Multi-stage cooling system with tandem compressors and optimized control of sensible cooling and dehumidification

ABSTRACT

A cooling system has a plurality of separate cooling stages including an upstream cooling stage having an upstream cooling circuit and a downstream cooling stage including a downstream cooling circuit, which are each a direct expansion cooling circuit including a tandem compressor. Each tandem compressor includes a fixed capacity compressor and a variable capacity compressor. A controller controls the fixed capacity compressor and variable capacity compressor of each tandem compressor based on a Call for Cooling, which of a plurality of ranges the Call for Cooling falls within, and whether the Call for Cooling is ramping up or ramping down.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Nos.61/476783, filed on Apr. 19, 2011 and 61/527695, filed on Aug. 26, 2011.The entire disclosures of each of the above applications areincorporated herein by reference.

FIELD

The present disclosure relates to cooling systems, and moreparticularly, to high efficiency cooling systems.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

Cooling systems have applicability in a number of different applicationswhere fluid is to be cooled. They are used in cooling gas, such as air,and liquids, such as water. Two common examples are building HVAC(heating, ventilation, air conditioning) systems that are used for“comfort cooling,” that is, to cool spaces where people are present suchas offices, and data center climate control systems.

A data center is a room containing a collection of electronic equipment,such as computer servers. Data centers and the equipment containedtherein typically have optimal environmental operating conditions,temperature and humidity in particular. Cooling systems used for datacenters typically include climate control systems, usually implementedas part the control for the cooling system, to maintain the propertemperature and humidity in the data center.

FIG. 1 shows an example of a typical data center 100 having a climatecontrol system 102 (also known as a cooling system). Data center 100illustratively utilizes the “hot” and “cold” aisle approach whereequipment racks 104 are arranged to create hot aisles 106 and coldaisles 108. Data center 100 is also illustratively a raised floor datacenter having a raised floor 110 above a sub-floor 112. The spacebetween raised floor 110 and sub-floor 112 provides a supply air plenum114 for conditioned supply air (sometimes referred to as “cold” air)flowing from computer room air conditioners (“CRACs”) 116 of climatecontrol system 102 up through raised floor 110 into data center 100. Theconditioned supply air then flows into the fronts of equipment racks104, through the equipment (not shown) mounted in the equipment rackswhere it cools the equipment, and the hot air is then exhausted outthrough the backs of equipment racks 104, or the tops of racks 104. Invariations, the conditioned supply air flows into bottoms of the racksand is exhausted out of the backs of the racks 104 or the tops of theracks 104.

It should be understood that data center 100 may not have a raised floor110 nor plenum 114. In this case, the CRAC's 116 would draw in throughan air inlet (not shown) heated air from the data center, cool it, andexhaust it from an air outlet 117 shown in phantom in FIG. 1 back intothe data center. The CRACS 116 may, for example, be arranged in the rowsof the electronic equipment, may be disposed with their cool air supplyfacing respective cold aisles, or be disposed along walls of the datacenter.

In the example data center 100 shown in FIG. 1, data center 100 has adropped ceiling 118 where the space between dropped ceiling 118 andceiling 120 provides a hot air plenum 122 into which the hot airexhausted from equipment racks 104 is drawn and through which the hotair flows back to CRACs 116. A return air plenum (not shown) for eachCRAC 116 couples that CRAC 116 to plenum 122.

CRACs 116 may be chilled water CRACs or direct expansion (DX) CRACs.CRACs 116 are coupled to a heat rejection device 124 that providescooled liquid to CRACs 116. Heat rejection device 124 is a device thattransfers heat from the return fluid from CRACs 116 to a cooler medium,such as outside ambient air. Heat rejection device 124 may include airor liquid cooled heat exchangers. Heat rejection device 124 may also bea refrigeration condenser system, in which case a refrigerant isprovided to CRACs 116 and CRACs 116 may be phase change refrigerant airconditioning systems having refrigerant compressors, such as a DXsystem. Each CRAC 116 may include a control module 125 that controls theCRAC 116.

In an aspect, CRAC 116 includes a variable capacity compressor and mayfor example include a variable capacity compressor for each DX coolingcircuit of CRAC 116. It should be understood that CRAC 116 may, as isoften the case, have multiple DX cooling circuits. In an aspect, CRAC116 includes a capacity modulated type of compressor or a 4-stepsemi-hermetic compressor, such as those available from Emerson ClimateTechnologies, Liebert Corporation or the Carlyle division of UnitedTechnologies. CRAC 116 may also include one or more air moving units119, such as fans or blowers. The air moving units 119 may be providedin CRACs 116 or may additionally or alternatively be provided in supplyair plenum 114 as shown in phantom at 121. Air moving units 119, 121 mayillustratively have variable speed drives.

A typical CRAC 200 having a typical DX cooling circuit is shown in FIG.2. CRAC 200 has a cabinet 202 in which an evaporator 204 is disposed.Evaporator 204 may be a V-coil assembly. An air moving unit 206, such asa fan or squirrel cage blower, is also disposed in cabinet 202 andsituated to draw air through evaporator 204 from an inlet (not shown) ofcabinet 202, where it is cooled by evaporator 204, and direct the cooledair out of plenum 208. Evaporator 204, a compressor 210, a condenser 212and an expansion valve 214 are coupled together in known fashion in a DXrefrigeration circuit. A phase change refrigerant is circulated bycompressor 210 through condenser 212, expansion valve 214, evaporator204 and back to compressor 210. Condenser 212 may be any of a variety oftypes of condensers conventionally used in cooling systems, such as anair cooled condenser, a water cooled condenser, or glycol cooledcondenser. It should be understood that condenser 210 is often not partof the CRAC but is located elsewhere, such as outside the building inwhich the CRAC is located. Compressor 210 may be any of a variety oftypes of compressors conventionally used in DX refrigeration systems,such as a scroll compressor. When evaporator 204 is a V-coil or A-coilassembly, it typically has a cooling slab (or slabs) on each leg of theV or A, as applicable. Each cooling slab may, for example, be in aseparate cooling circuit with each cooling circuit having a separatecompressor. Alternatively, the fluid circuits in each slab such as wherethere are two slabs and two compressor circuits, can be intermingledamong the two compressor circuits.

Evaporator 204 is typically a fin-and-tube assembly and is used to bothcool and dehumidify the air passing through them. Typically, CRAC's suchas CRAC 200 are designed so that the sensible heat ratio (“SHR”) istypically between 0.85 and 0.95.

A system known as the GLYCOOL free-cooling system is available fromLiebert Corporation of Columbus, Ohio. In this system, a second coolingcoil assembly, known as a “free cooling coil,” is added to a CRAC havinga normal glycol system. This second coil assembly is added in the airstream ahead of the first cooling coil assembly. During colder months,the glycol solution returning from the outdoor drycooler is routed tothe second cooling coil assembly and becomes the primary source ofcooling to the data center. At ambient temperatures below 35 deg. F, thecooling capacity of the second cooling coil assembly is sufficient tohandle the total cooling needs of the data center and substantiallyreduces energy costs since the compressor of the CRAC need not be run.The second or free cooling coil assembly does not provide 100% sensiblecooling and has an airside pressure drop similar to the evaporator(which is the first cooling coil assembly).

Efficiency of cooling systems has taken on increased importance.According to the U.S. Department of Energy, cooling and power conversionsystems for data centers consume at least half the power used in atypical data center. In other words, less than half the power isconsumed by the servers in the data center. This has led to increasedfocus on energy efficiency in data center cooling systems.

SUMMARY

In accordance with an aspect of the present disclosure, a cooling systemincludes a cabinet having an air inlet and an air outlet and a coolingcircuit that includes an evaporator disposed in the cabinet, acondenser, a compressor, an expansion device and a liquid pump. Thecooling system has a direct expansion mode wherein the compressor is onand compresses a refrigerant in a vapor phase to raise its pressure andthus its condensing temperature and refrigerant is circulated around thecooling circuit by the compressor. The cooling system also has a pumpedrefrigerant economizer mode wherein the compressor is off and the liquidpump is on and pumps the refrigerant in a liquid phase and refrigerantis circulated around the cooling circuit by the liquid pump and withoutcompressing the refrigerant in its vapor phase. In an aspect, thecooling system has a controller coupled to the liquid pump and thecompressor that turns the compressor off and the liquid pump on tooperate the cooling circuit in the economizer mode and turns thecompressor on to operate the cooling circuit in the direct expansionmode. In an aspect, the controller turns the liquid pump off when thecooling circuit is in the direct expansion mode. In an aspect, theexpansion device is an electronic expansion valve.

In an aspect, the cooling circuit includes a receiver/surge tank coupledbetween the condenser and the liquid pump.

In an aspect, the cooling system includes a plurality of coolingcircuits with each cooling circuit included in one of a plurality ofcooling stages including an upstream cooling stage and a downstreamcooling stage wherein the evaporator of the cooling circuit of theupstream cooling stage (upstream evaporator) and the evaporator of thecooling circuit of the downstream cooling stage (downstream evaporator)are arranged in the cabinet so that air to be cooled passes over them inserial fashion, first over the upstream evaporator and then over thedownstream evaporators. The cooling circuit of each cooling stage hasthe direct expansion mode wherein the compressor of that cooling circuitis on and the refrigerant is circulated around the cooling circuit bythe compressor of that cooling circuit and a pumped refrigeranteconomizer mode wherein the compressor of that cooling circuit is offand the liquid pump of that cooling circuit is on and the refrigerant iscirculated around the cooling circuit by the liquid pump of that coolingcircuit. In an aspect, when one of the upstream and downstream coolingstages can be in the economizer mode and the other must be in directexpansion mode, the controller operates the cooling circuit of theupstream cooling stage in the economizer mode turning liquid pump ofthat cooling circuit on and the compressor of that circuit off andoperates the downstream cooling stage in the direct expansion modeturning the compressor of the downstream cooling circuit on.

In an aspect, a cooling system includes a cabinet having an air inletand an air outlet and a cooling circuit that includes a direct expansionrefrigeration cooling circuit including an evaporator disposed in thecabinet, a condenser, a compressor and an expansion device wherein thecondenser is at an elevation higher than the evaporator. The coolingcircuit has a direct expansion mode wherein the compressor is on andcompresses a refrigerant in a vapor phase to raise its pressure and thusits condensing temperature and refrigerant is circulated around thecooling circuit by the compressor and an economizer mode wherein thecompressor is off and a liquid column of refrigerant at an inlet of theevaporator induces a thermo-siphon effect causing refrigerant tocirculate around the cooling circuit and without compressing therefrigerant in its vapor phase. In an aspect, a controller is coupled tothe compressor that turns the compressor off to operate the coolingcircuit in the economizer mode and turns the compressor on to operatethe cooling circuit in the direct expansion mode.

In an aspect, a cooling system has a cabinet having an air inlet and anair outlet and a cooling circuit that includes an evaporator disposed inthe cabinet, a condenser, a compressor, a liquid/vapor separator tankand a liquid pump. The cooling circuit has a mode wherein the compressorand liquid pump are both on with the liquid pump pumping refrigerantthrough the evaporator with the refrigerant leaving the evaporatorcirculated to an inlet of the liquid/vapor separator tank and not to aninlet of the condenser, and the compressor compressing refrigerantcirculating to an inlet of the compressor from an outlet of theliquid/vapor separator tank to raise its pressure and thus itscondensing temperature with refrigerant leaving the compressorcirculated to the inlet of the condenser. The cooling circuit also has apumped refrigerant economizer mode wherein the liquid pump is on and thecompressor is off and bypassed, the liquid pump pumping refrigerant in aliquid phase through the evaporator with the refrigerant leaving theevaporator circulated to the inlet of the condenser and not to the inletof the liquid/vapor separator tank and wherein refrigerant circulateswithout compression of the refrigerant in its vapor phase.

In an aspect, a cooling system has a cabinet having an air inlet and anair outlet. The cooling system includes a direct expansion coolingcircuit and a pumped cooling fluid cooling circuit. The direct expansioncooling circuit includes an evaporator disposed in the cabinet, acondenser, a compressor and an expansion device. The pumped coolingfluid cooling circuit includes an evaporator disposed in the cabinet, acondenser, a liquid pump and an expansion device. The evaporators arearranged in the cabinet so that air flows over them in serial fashionwith the cooling circuit having the most upstream evaporator being avariable capacity cooling circuit and an upstream cooling circuit. Thecooling system has a direct expansion mode wherein the direct expansioncooling circuit is operating to provide cooling and a pumped coolingfluid economizer mode wherein the direct expansion cooling circuit isnot operating to provide cooling and the pumped cooling fluid coolingcircuit is operating to provide cooling. In an aspect, when the coolingsystem is in the direct expansion mode, the pumped cooling fluid coolingcircuit is also operated to provide cooling. A controller controls theoperation of the cooling circuits. The controller when a Call forCooling first reaches a point where cooling is needed, operating theupstream cooling circuit to provide cooling and not operating thedownstream cooling circuit to provide cooling and when the Call forCooling has increased to a second point, additionally operating thedownstream cooling circuit to provide cooling, wherein the coolingcapacity at which the upstream cooling circuit is being operated toprovide is less than the full cooling capacity of the upstream coolingcircuit when the Call for Cooling reaches the second point. In anaspect, the pumped cooling fluid cooling circuit is the upstream coolingcircuit.

In an aspect, the expansion device of each cooling circuit having apumped refrigerant economizer mode is an electronic expansion valve andwhen any cooling circuit having the pumped refrigerant economizer modeis in the pumped refrigerant economizer mode, the controller of thecooling system controls a temperature of the refrigerant to arefrigerant temperature set point by regulating a speed of a fan of thecondenser of the cooling circuit, controls a temperature of air in aroom in which the cabinet is disposed to a room air temperature setpointby regulating a speed of the liquid pump of the cooling circuit, andmaintains a pressure differential across the liquid pump of the coolingcircuit within a given range by regulating an open position of theelectronic expansion valve of the cooling circuit.

In an aspect, the controller for the control of the pumped refrigeranteconomizer mode of a cooling circuit has a refrigerant temperaturefeedback control loop for controlling the temperature of the refrigerantof that cooling circuit by regulating the speed of the condenser fan ofthat cooling circuit, a room air temperature feedback control loop forcontrolling the temperature of the air in the room in which the cabinetis disposed by regulating the speed of the liquid pump of that coolingcircuit, and a liquid pump pressure differential control feedback loopfor controlling a pressure differential across the liquid pump of thatcooling circuit by regulating a position of the electronic expansionvalve of that cooling circuit. In an aspect, the controller has aseparate controller for each of the feedback control loops. In anaspect, the refrigerant temperature set point is a fixed set point, theroom air temperature set point is a user input setpoint that the userinputs into the controller, and the given range is a fixed range. In anaspect, the refrigerant temperature control loop also includes as aninput an output of a feed forward controller and the feed forwardcontroller has as inputs a liquid pump speed control signal from theroom air temperature feedback control loop and an electronic expansionvalve position signal from an output of the liquid pump pressuredifferential control feedback loop.

In accordance with an aspect, a cooling system has a cabinet having anair inlet and an air outlet, an air moving unit disposed in the cabinet,and a plurality of separate cooling stages including an upstream coolingstage and a downstream cooling stage. Each cooling stage includes acooling circuit having an evaporator, a condenser, a tandem digitalscroll compressor and an expansion device. Each tandem compressorincludes a fixed capacity compressor and variable capacity digitalscroll compressor. At least the cooling circuit of the upstream coolingstage has a pumped refrigerant economizer mode and a direct expansionmode. Each cooling circuit that has both the pumped refrigeranteconomizer mode and the direct expansion mode also has a liquid pumpwherein when that cooling circuit is operated in the direct expansionmode the compressor of that cooling circuit is on and compresses arefrigerant in a vapor phase to raise its pressure and thus itscondensing temperature and refrigerant is circulated around the coolingcircuit by the compressor and wherein when that cooling circuit isoperated in the pumped refrigerant economizer mode the compressor ofthat cooling circuit is off and the liquid pump of that cooling circuitis on and pumps the refrigerant in a liquid phase and refrigerant iscirculated around that cooling circuit by the liquid pump of thatcooling circuit and without compressing the refrigerant in its vaporphase. The evaporator of the cooling circuit of the upstream coolingstage (upstream evaporator) and the evaporator of the cooling circuit ofthe downstream cooling stage (downstream evaporator) are arranged in thecabinet so that air to be cooled passes over them in serial fashion,first over the upstream evaporator and then over the downstreamevaporator. The cooling system includes a controller that determineswhich of the cooling circuits to operate to provide cooling and for eachof the cooling circuits to be operated to provide cooling that has boththe pumped refrigerant economizer mode and direct expansion mode,determines whether to operate each such cooling circuit in the pumpedrefrigerant economizer mode or the direct expansion mode. The controlleroperating each cooling circuit having both the pumped refrigeranteconomizer mode and the direct expansion mode in the pumped refrigeranteconomizer mode when an outside air temperature is low enough to providesufficient heat rejection from the refrigerant flowing through thecondenser to the outside air without compressing the refrigerant andwhen the outside air temperature is not low enough to provide suchsufficient heat rejection operating that cooling circuit in the directexpansion mode. The controller when any of the cooling circuits arebeing operated in the direct expansion mode controlling the electronicexpansion valve of that cooling circuit to control a suction superheatof the evaporator of that cooling circuit. The controller when any ofthe cooling circuits having both the pumped refrigerant economizer modeand the direct expansion mode is being operated in the pumpedrefrigerant economizer mode controlling the expansion device of thatcooling circuit to maintain a minimum differential pressure across theliquid pump of that cooling circuit. In an aspect, each cooling circuithas both the pumped refrigerant economizer mode and the direct expansionmode.

In an aspect, a cooling system has a cabinet having an air inlet and anair outlet. An air moving unit is disposed in the cabinet. an air movingunit disposed in the cabinet. The cooling system has a plurality ofseparate cooling stages including an upstream cooling stage and adownstream cooling stage with at least the upstream cooling stage avariable capacity cooling circuit, Each cooling stage including acooling circuit having an evaporator, a condenser, a compressor and anexpansion device. At least the cooling circuit of the upstream coolingstage having a pumped refrigerant economizer mode and a direct expansionmode wherein each cooling circuit that has both the pumped refrigeranteconomizer mode and the direct expansion mode also has a liquid pumpwherein when that cooling circuit is operated in the direct expansionmode a compressor of that cooling circuit is on and compresses arefrigerant in a vapor phase to raise its pressure and thus itscondensing temperature and refrigerant is circulated around the coolingcircuit by the compressor of that cooling circuit and wherein when thatcooling circuit is operated in the pumped refrigerant economizer modethe compressor of that cooling circuit is off and the liquid pump ofthat cooling circuit is on and pumps the refrigerant in a liquid phaseand refrigerant is circulated around that cooling circuit by the liquidpump of that cooling circuit and without compressing the refrigerant inits vapor phase. The evaporator of the cooling circuit of the upstreamcooling stage (upstream evaporator) and the evaporator of the coolingcircuit of the downstream cooling stage (downstream evaporator) arrangedin the cabinet so that air to be cooled passes over them in serialfashion, first over the upstream evaporator and then over the downstreamevaporator. A controller that determines which of the cooling circuitsto operate to provide cooling and for each of the cooling circuits to beoperated to provide cooling that has both the pumped refrigeranteconomizer mode and direct expansion mode, determining whether tooperate each such cooling circuit in the pumped refrigerant economizermode or the direct expansion mode. The controller operating each coolingcircuit having both the pumped refrigerant economizer mode and thedirect expansion mode in the pumped refrigerant economizer mode when anoutside air temperature is low enough to provide sufficient heatrejection from the refrigerant flowing through the condenser to theoutside air without compressing the refrigerant and when the outside airtemperature is not low enough to provide such sufficient heat rejectionoperating that cooling circuit in the direct expansion mode. Thecontroller when a Call for Cooling first reaches a point where coolingis needed, operating the upstream cooling circuit to provide cooling andnot operating the downstream cooling circuit to provide cooling and whenthe Call for Cooling has increased to a second point, additionallyoperating the downstream cooling circuit to provide cooling, wherein thecooling capacity at which the upstream cooling circuit is being operatedto provide is less than the full cooling capacity of the upstreamcooling circuit when the Call for Cooling reaches the second point.

In an aspect, the condensers of each cooling circuit include anelectronically commutated fan. The controller of the cooling systemvaries the speed of the electronically commutated fan to maintain atemperature of the refrigerant leaving the condenser at a setpoint.

In an aspect, the air moving unit includes at least one electronicallycommutated fan. The controller of the cooling system increases the speedof the electronically commutated fan as a cooling load on the coolingsystem increases and decreases the speed of the electronicallycommutated fan as the cooling load decreases.

In an aspect, the controller of the cooling system operates the fixedcapacity compressor and variable capacity digital scroll compressor ofeach tandem compressor to maximize operation of the variable capacitydigital scroll compressor in an upper loading range of the variablecapacity digital scroll compressor.

In an aspect, the controller of the cooling system determines which ofthe cooling circuits to operate in a direct expansion mode to providecooling based on a Call for Cooling. When the cooling circuits are beingoperated in the direct expansion mode, the controller controls the fixedcompressor and variable capacity digital scroll compressor of the tandemdigital scroll compressor of that cooling circuit based on the Call forCooling, which of a plurality of ranges that the Call for Cooling fallswithin and whether the Call for Cooling is ramping up or ramping down.In an aspect, the controller first begins ramping the variable capacitydigital scroll compressor of the cooling circuit of the upstream coolingstage to operate the upstream cooling stage to provide cooling and thenas the Call for Cooling increases above a threshold, also begins rampingthe variable capacity digital scroll compressor of the cooling circuitof the downstream cooling stage in parallel with ramping the variablecapacity digital scroll compressor of the upstream cooling circuit tooperate both the upstream cooling stage and the downstream cooling stageto provide cooling.

In an aspect, the controller controls the fixed compressor and variablecapacity digital scroll compressor of each tandem digital scrollcompressor based on a Call for Cooling Call and a Call forDehumidification which takes precedence over control based on only theCall for Cooling when there is an unmet Call for Dehumidification.

In an aspect, the controller when a cooling circuit is operated in thedirect expansion mode controls the electronic expansion valve of thatcooling circuit to control a suction superheat of the evaporator of thatcooling circuit and the controller when a cooling circuit having boththe pumped refrigerant economizer mode and the direct expansion mode isbeing operated in the pumped refrigerant economizer mode controlling theexpansion valve of that cooling circuit to maintain a minimumdifferential pressure across the liquid pump of that cooling circuit.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a schematic illustrating a prior art data center;

FIG. 2 is a simplified perspective view of a prior art CRAC having a DXcooling circuit;

FIG. 3 is a schematic showing a CRAC having staged cooling provided bytwo cooling circuits in accordance with an aspect of the presentdisclosure;

FIG. 4 is a simplified perspective view of a CRAC having the coolingcircuits of the CRAC of FIG. 3;

FIG. 5 is a simplified perspective view of another CRAC having thecooling circuits of the CRAC of FIG. 3;

FIG. 6 is a schematic showing a CRAC having staged cooling provided bythree cooling circuits in accordance with an aspect of the presentdisclosure;

FIG. 7 is a simplified perspective view showing a CRAC having stagedcooling provided by two cooling circuits with each cooling circuithaving a tandem digital scroll compressor in accordance with an aspectof the present disclosure;

FIG. 8 is a simplified perspective view of evaporators having coolingslabs arranged in an interleaved configuration in accordance with anaspect of the present disclosure;

FIG. 9 is a simplified perspective view of a variation of the CRAC ofFIG. 7 where one of the cooling circuits includes a suction line heatexchanger in accordance with an aspect of the present disclosure;

FIG. 10 is a simplified perspective view of the CRAC of FIG. 10 whereboth cooling circuits include a suction line heat exchanger inaccordance with an aspect of the present disclosure;

FIGS. 11A and 11B are tables showing control settings for tandem digitalscroll compressors used in a CRAC having the staged cooling circuits forsensible cooling control and for dehumidification control in accordancewith an aspect of the present disclosure and FIG. 11C is a flow chartshowing this control;

FIG. 12 is a cooling system having a DX cooling circuit with a pumpedrefrigerant economizer mode in accordance with an aspect of the presentdisclosure;

FIGS. 13-24 are variations of the cooling system of FIG. 12;

FIG. 25 is a schematic showing a cooling system having a DX coolingcircuit and a separate pumped refrigerant economizer circuit inaccordance with an aspect of the present disclosure;

FIG. 26 is a schematic showing a cooling system having staged coolingprovided by two cooling circuits of FIG. 12 in accordance with an aspectof the present disclosure;

FIG. 27 is a schematic showing the cooling system of FIG. 12 and showingin more detail the control system therewith;

FIG. 28 shows control loops for the control system of FIG. 27;

FIG. 29 is a flow chart showing an illustrative control of an electronicexpansion valve in accordance with an aspect of the present disclosure;and

FIG. 30 is a flow chart showing an illustrative control of a coolingsystem having staged cooling to stage based on a Call for Cooling tostage the operation of cooling circuits of the cooling system.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

In accordance with an aspect of the present disclosure, a highefficiency cooling system includes staged cooling provided by two ormore cooling circuits arranged so that air to be cooled flows throughthem serially. In an aspect, each cooling circuit includes a tandemdigital scroll compressor made up of a fixed capacity scroll compressorand digital scroll compressor. It should be understood that instead oftandem digital compressors, a plurality of compressors can be plumbed inparallel and these compressors may have differing capacities. In anaspect, each cooling circuit includes a DX cooling circuit and a pumpedrefrigerant economization circuit that bypasses the compressor when theoutdoor temperature is sufficiently low to provide the requisite coolingto the refrigerant being circulating in the cooling circuit. In anaspect, the high efficiency cooling system also includes one or morefans, blowers or similar air moving units that move air to be cooledthrough the evaporators of each cooling circuit. The motors of the airmoving unit may illustratively be variable speed motors, and mayillustratively be electronically controlled motors. The same may be thecase for the fan motors for the condenser. In an aspect, the coolingcircuits of the high efficiency cooling system include an electronicexpansion valve.

It should be understood that a cooling system can have less than allthese elements, and can have various combinations of them. For example,the cooling system may not have staged cooling but have a coolingcircuit that includes a DX cooling circuit and the pumped refrigeranteconomization circuit. In this aspect, the tandem digital scroll may ormay not be utilized.

FIG. 3 is a simplified schematic of a cooling system 300 having aplurality of cooling stages including an upstream cooling stage 322 withan upstream cooling circuit 301 and a downstream cooling stage 324 witha downstream cooling circuit 302 in accordance with an aspect of thepresent disclosure. In the embodiment of FIG. 3, cooling circuits 301,302 are both DX refrigeration circuits. Upstream cooling circuit 301includes an evaporator referred to as upstream evaporator 304, expansionvalve 306, condenser 308 and compressor 310 arranged in a conventionalDX refrigeration circuit. Downstream cooling circuit 302 includes anevaporator referred to as downstream evaporator 312, expansion valve314, condenser 316 and compressor 318 arranged in a conventional DXrefrigeration circuit. In this regard, evaporator 304, expansion valve306 and compressor 310 of upstream cooling circuit 301 and evaporator312, expansion valve 314 and compressor 318 of downstream coolingcircuit 302 may all be included in a CRAC 326 located in a data centeralong with controller 320. Condensers 308, 316 are shown in dashed boxesas they are typically not included in CRAC 326 but located elsewhere,such as outside the building in which CRAC 326 is located. Expansionvalves 306, 314 may preferably be electronic expansion valves, but mayalso be thermostatic expansion valves such as those disclosed in U.S.Pat. No. 4,606,198. In each DX refrigeration circuit 301, 302, arefrigerant is circulated by the compressor and it flows from thecompressor, through the condenser, expansion valve, evaporator and backto the compressor. The evaporators 304, 312 of upstream and downstreamcooling circuits 301, 302 are arranged in stages so that air drawn inthrough an inlet of the CRAC flows in serial fashion through evaporators304, 312, that is, the air flows first through the upstream evaporator304 in upstream cooling circuit 301 and then through downstreamevaporator 312 in the downstream cooling circuit 302. By having aplurality of cooling stages arranged for serial air flow therethrough,the temperature differential across the evaporators of each DXrefrigeration circuit is reduced. This in turn allows the evaporators ineach DX refrigeration circuit to operate at different pressure levelsand allows the pressure differences between the respective evaporatorsand condensers to be reduced. Since compressor power is a function ofthe pressure difference between the evaporator and condenser, a lowerpressure difference is more energy efficient. It should be understoodthat each compressor 310, 318 may include tandem compressors with onecompressor a fixed capacity compressor and the other compressor avariable capacity compressor, such as a digital scroll compressor. Eachcompressor 310, 318 may be a tandem digital scroll compressor thatincludes a fixed capacity scroll compressor and a digital scrollcompressor, as discussed in more detail below.

It should be understood that condensers 308, 316 can be any of the heatrejection devices described above with regard to heat rejection device124 of FIG. 1.

The cooling circuit of each stage provides a portion of the overallcooling provided by CRAC 326 of cooling system 300. The portions can beequal, with each stage providing equal cooling, or they can bedifferent. More specifically, each cooling stage has a maximumtemperature difference that is a portion of the maximum temperaturedifference across CRAC 326. For example, if CRAC 326 has a maximumtemperature difference of 20 deg. F, the cooling circuit of each stagehas a maximum temperature difference that is some percentage of 20 deg.F. This may be an equal percentage, in which case cooling circuit 301,302 each have a maximum 10 deg. F temperature difference where themaximum temperature difference across CRAC 326 is 20 deg. F, or thepercentages may be different.

Cooling system includes controller 320 that controls cooling circuits301, 302.

Upstream evaporator 304 of upstream cooling circuit 301 sees higherinlet air temperatures and compressor 310 of upstream cooling circuit301 supplies refrigerant to upstream evaporator 304 at a higherevaporating temperature than that supplied by compressor 318 todownstream evaporator 312 in downstream cooling circuit 302. Downstreamevaporator 312 in downstream cooling circuit 302 sees the lower airtemperature exiting evaporator 304 of upstream cooling circuit 301.Compared to current technology, there is an optimal point, along acontinuum of cooling from cooling only by downstream cooling circuit 302to cooling only by upstream cooling circuit 301 at which the same nettotal cooling capacity is achieved with smaller compressors in theupstream and downstream cooling circuits 301, 302, with upstream anddownstream cooling circuits 301, 302 and evaporators 304, 312 ofupstream and downstream cooling circuits 301, 302 configured to provideapproximately equal cooling capacity. For example, if CRAC 326 is a 30ton unit, cooling circuits 301, 302 would each be configured to provideapproximately 15 tons of cooling capacity as would evaporators 304, 312.Evaporators 304, 312 are configured to have approximately equal surfacecooling area (the cooling surface area being the area contacted by theair flowing through the evaporator). In this regard, when evaporators304, 312 have a plurality of cooling slabs, such as in a V-coilassembly, instead of having each cooling slab of downstream evaporator312 be fed by separate compressors, both cooling slabs of downstreamevaporator 312 would be fed by a compressor and both cooling slabs ofupstream evaporator 304 would be fed by another compressor. These twocompressors would preferably have equal capacity and the staged coolingallows the two compressors to be smaller (lesser capacity) than the twocompressors used to feed the two cooling slabs of an evaporator in atypical prior art CRAC having DX refrigeration circuits for the twocooling slabs that provide comparable cooling capacity.

In an alternate embodiment, compressor 318 in downstream cooling circuit302 is larger (that is, has a higher capacity) than compressor 310 inupstream cooling circuit 301 in order to decrease the evaporatingtemperature of the refrigerant provided to downstream evaporator 312.This in turn decreases the sensible heat ratio and increases thedehumidification capabilities of downstream cooling circuit 302. In thisembodiment, downstream evaporator 312 may have the same cooling surfacearea as that of upstream evaporator 304 in upstream cooling circuit 301,or may have a cooling surface area that is different (larger or smaller)than the surface cooling area of upstream evaporator 304.

In an aspect, upstream evaporator 304 in upstream cooling circuit 301 isa microchannel cooling coil assembly. Upstream evaporator 304 mayillustratively be a microchannel heat exchanger of the type described inU.S. Ser. No. 12/388,102 filed Feb. 18, 2009 for “Laminated Manifold forMicrochannel Heat Exchanger” the entire disclosure of which isincorporated herein by reference. Upstream evaporator 304 mayillustratively be a MCHX microchannel heat exchanger available fromLiebert Corporation of Columbus, Ohio. When upstream evaporator 304 is amicro-channel heat exchanger, upstream cooling circuit 301 isillustratively configured to provide sensible only cooling, such asproviding a temperature delta across upstream evaporator 304 that doesnot drop the temperature of the air exiting upstream evaporator 304below its dewpoint, or below a temperature a certain number of degreesabove the dewpoint, such as about 4 deg. F. While one advantage of usinga microchannel cooling coil assembly for upstream evaporator 304 ofupstream cooling circuit 301 is that microchannel cooling coilassemblies have air side pressure drops across them that aresignificantly less than fin-and-tube cooling coil assemblies havingcomparable cooling capacity, it should be understood that upstreamevaporator 304 can be other than a microchannel cooling coil, and mayfor example be a fin-and-tube cooling coil assembly.

In an aspect, downstream evaporator 312 of downstream cooling circuit302 is a fin-and-tube cooling coil assembly. In an aspect, downstreamevaporator 312 is a microchannel cooling coil assembly.

FIG. 4 shows an illustrative embodiment of CRAC 326. CRAC 326 includes acabinet 400 having a return air inlet 402 and an air outlet 404, such asa plenum. An air filter 406 is disposed at return air inlet 402 so thatair flowing into CRAC 326 through return air inlet 402 flows through airfilter 406 before flowing through the rest of CRAC 326. Arrows 414 showthe direction of air flow through CRAC 326.

In the embodiment shown in FIG. 4, downstream evaporator 312 ofdownstream cooling circuit 302 is an A-coil assembly disposed in cabinet400 between return air inlet 402 and air outlet 404. Downstreamevaporator 312 thus has a cooling slab 410 for each leg of the A.Upstream evaporator 304 is also an A-coil assembly having a cooling slab412 for each leg of the A. An air moving unit 408, such as a fan orsquirrel cage blower, is disposed in cabinet 400 between a downstreamside of downstream evaporator 312 and air outlet 404. One of the coolingslabs 412 of upstream evaporator 304 is disposed on the air inlet sideof one of the cooling slabs 410 of downstream evaporator 312 and theother of the cooling slabs 412 of upstream evaporator 304 is disposed onthe air inlet side of the other of the cooling slabs 410 of downstreamevaporator 312. The cooling slabs 410 of downstream evaporator 312 andthe cooling slabs 412 of upstream evaporator 304 are thus arranged inpairs, with respective ones of the cooling slabs 412 of upstreamevaporator paired with respective ones of the cooling slabs 410 ofdownstream evaporator 312. In should be understood that air moving unit408 may alternatively be disposed upstream of upstream evaporator 304.

Alternatively, as shown in FIG. 5, upstream evaporator 304′ in upstreamcooling circuit 301 may be disposed in a plenum 415 of cabinet 400between air filter 406 and downstream evaporator 312.

In a variation, the cooling slabs 412 of upstream evaporator 304 couldbe segmented into multiple cooling slabs, as could cooling slabs 410 ofdownstream evaporator 312.

Staging the cooling in the CRAC with upstream and downstream separate DXrefrigeration circuits allows the pressure difference across thecompressor of the upstream DX refrigeration circuit to be reduced,thereby reducing its power consumption. The additional surface areaprovided by the upstream evaporator in the upstream DX refrigerationcircuit allows the temperature delta across the downstream evaporator inthe downstream DX refrigeration circuit to be reduced. This allows thepressure difference across the compressor in the downstream DXrefrigeration circuit to be reduced, thereby reducing its powerconsumption. The staging also elevates the temperature of theevaporators so that they do less dehumidification. In a data center,dehumidification is typically a waste of energy. Staging the cooling hasthe further benefit of enabling the CRAC to accommodate large air sidetemperature differences from inlet to outlet. The combination of theseeffects greatly increases the SHR.

The compressor of a DX cooling circuit runs more efficiently and withgreater capacity when the difference between evaporating and condensingpressures is reduced. In addition, if increased energy efficiency asopposed to greater capacity is the objective, then the compressors canbe smaller and still meet the desired mass flow rate for the refrigerantflowing through the cooling circuit since the evaporating temperaturehas been raised. That is, the compressors in each circuit can be smallerthan the compressors used to feed the cooling slabs of a cooling coil ina typical prior art CRAC having DX refrigeration circuits for eachcooling slab and still achieve the same net total cooling capacity.

It should be understood that cooling system 300 could have more than twostaged cooling circuits, with each staged cooling circuit illustrativelybeing a DX cooling circuit such as cooling circuit 301, 302. Forexample, a cooling system such as cooling system 600 in FIG. 6 has threestaged cooling circuits, with the third cooling circuit 602 arrangeddownstream of cooling circuit 302. Each stage may then provide an equal(i.e., ⅓) portion of the cooling provided by cooling system 600, or eachstage may provide different portions.

As mentioned above, each compressor 310, 318 may be a tandem compressorsuch as a tandem compressor known as a tandem digital scroll compressorthat includes both a fixed capacity scroll compressor and a variablecapacity digital scroll compressor. As used herein, “tandem digitalscroll compressor” means a compressor that has both a fixed capacityscroll compressor and a variable capacity digital scroll compressor.FIG. 7 shows a CRAC 700 that is a variation of CRAC 326 (FIG. 3) withtandem digital scroll compressors 710 and 718 with tandem digital scrollcompressor 710 including a fixed capacity compressor 710(F) and avariable capacity digital scroll compressor 710(V), and tandem digitalscroll compressor 718 including a fixed capacity compressor 718(F) and avariable capacity digital scroll compressor 718(V). Fixed capacitycompressors 710(F) and 718(F) may preferably be fixed capacity scrollcompressors, but it should be understood that they can be other types offixed capacity compressors. A digital scroll compressor has the abilityto vary or modulate its capacity between about 10% and 100% byseparation of scroll sets. The digital scroll compressor has lowerefficiency when operating at a part load condition and more efficientwhen operating at a higher load condition. More specifically, digitalscroll compressors tend to be more efficient when operating between 50%and 100% of capacity (i.e., loaded between 50% and 100%) and lessefficient when operating below 50% of capacity. Pairing a fixed capacitycompressor with a digital scroll compressor in a tandem digital scrollprovides a broader range of energy efficient operation from about 25% to100% of capacity. Tandem digital scroll compressors may illustrativelybe tandem digital scroll compressors available from Emerson ClimateTechnology, Sydney Ohio, under the Copeland® brand. As used herein,upper loaded range means the loading range of a certain loadingpercentage and above where a digital scroll compressor operates moreefficiently. While it is typically 50% or higher, it should beunderstood that the lower loading can differ from 50% and depend on theparticular compressor.

Upstream and downstream evaporators 304, 312 may have variousconfigurations. They may each have, for example, two cooling slabs havemultiple rows of coils through which the coolant flows. They may also beseparate from each other, as shown in FIGS. 4 and 7, or have aninterleaved configuration where rows of coils of cooling slabs 412, 410of upstream and downstream evaporators 304, 312 are interleaved witheach other as shown in FIG. 8.

In the illustrative configuration of FIG. 7, upstream and downstreamevaporators 304, 312 arranged as shown in FIG. 4 in a configurationreferred to herein as a “separate configuration” where they are separatefrom each other and upstream evaporator 304 is wholly upstream ofdownstream evaporator 312. That is, cooling slabs 412 of upstreamevaporator 304 are separate from cooling slabs 410 of downstreamevaporator 312 and cooling slabs 412 are wholly upstream of coolingslabs 410. Cooling slabs 410, 412 are arranged in pairs as discussedabove with an outlet side 702 of one of cooling slabs 412 of upstreamevaporator 304 abutting an inlet side 704 of one of cooling slabs 410 ofdownstream evaporator 312. In the example configuration shown in FIGS. 4and 7, cooling slabs 412 of upstream evaporator 304 are outboard ofcooling slabs 410 of downstream evaporator 312.

Cooling slabs 410, 412 may, for example, each have three rows 706 ofcoils 708 through which the refrigerant flows. The rows 706 of coils 708in cooling slabs 412 of upstream evaporator 304 are grouped separatelyfrom the rows 706 of coils 708 in cooling slabs 410 of downstreamevaporator 312. Thus, the rows 706 of coils 708 in cooling slabs 412 ofupstream evaporator 304 are all disposed upstream of the rows 706 ofcoils 708 in cooling slabs 410 of downstream evaporator 312. Thisconfiguration may be referred to herein as an “X row/X row—Z StageSeparate” configuration where X is the number of rows 706 of coils 708in a cooling slab and Z is the number of cooling stages. The exampleembodiment shown in FIG. 7 is thus a 3 row/3 row—2 Stage separateconfiguration. It should be understood that each cooling slab can havemore or less than 3 rows of coils.

FIG. 8 shows a configuration referred to herein as an “interleavedconfiguration” where one or more rows of coils of the upstreamevaporator 304 and downstream evaporator 312 are interleaved. In theexample interleaved configuration shown in FIG. 8, cooling slabs 410′,412′ of evaporators 304, 312, respectively, are arranged in pairs. Eachcooling slab 410′, 412′ has two sections—a superheat section and a2-phase section. For reference purposes, 412′ of upstream evaporator 304has superheat section 800 and 2-phase section 802 and each cooling slab410′ of downstream evaporator 312 has superheat section 804 and 2-phasesection 806. Each pair of cooling slabs 410′, 412′ are arranged suchthat the superheat section 804 of cooling slab 410′ of downstreamevaporator 304 is disposed between superheat section 800 of cooling slab412′ of upstream evaporator 304 and 2-phase section 802 of cooling slab412′ of upstream evaporator 304. Superheat section 800 of cooling slab412′ of upstream evaporator 304 has an inlet side 808 facing outboardlyas shown in FIG. 8 and an outlet side 810 facing an inlet side 812 ofsuperheat section 804 of cooling slab 410′ of downstream evaporator 312.An outlet side 814 of superheat section 804 of cooling slab 410′ ofdownstream evaporator 312 faces an inlet side 816 of 2-phase section 802of cooling slab 412′ of upstream evaporator 304. An outlet side 818 of2-phase section 802 faces an inlet side 820 of 2-phase section 806 ofcooling slab 410′ of downstream evaporator 312. In this configuration,air to be cooled enters superheat section 800 of cooling slab 412′ ofupstream cooling evaporator 304 and then passes through in sequencesuperheat section 804 of cooling slab 410′ of downstream evaporator 312,2-phase section 802 of cooling slab 412′ of upstream evaporator 304, and2-phase section 806 of cooling slab 410′ of downstream evaporator 312.This configuration may be referred to herein as a “X row/X row, Y row/Yrow—Z Stage interleaved” configuration where X is the number of rows inthe superheat section of a cooling slab, Y is the number of rows in the2-phase section of a cooling slab, and Z is the number of coolingstages. The example embodiment shown in FIG. 8 is thus a 1-row/1-row,2-row/2-row—2 Stage interleaved configuration. It should be understoodthat the 2-phase section of each cooling slab can have more or less than2 rows of coils and the superheat section of each cooling slab can havemore than one row of coils.

In the interleaved configuration, refrigerant in each of the respectiveupstream and downstream cooling stages first flows through the 2-phasesection of each cooling slab of the evaporator of that cooling stage andthen through the superheat section of that cooling slab. The refrigerantwill typically enter the 2-phase section in two phases (liquid and gas)and will typically exit the 2-phase section only as a gas. Therefrigerant is then superheated in the superheat section, which seeshotter air than the 2-phase sections.

Evaporating temperature for a multi-stage cooling system of the typesdescribed above is constrained by the superheat temperature, especiallyfor the downstream stage(s). By separating the superheat region in theinterleaved configuration to the entering air side, the superheatlimitation for the second stage is eliminated and the evaporatingtemperature of the second stage increases compared to a configurationwhere the coils of the evaporators are not interleaved—that is, thecoils of the upstream evaporator are all upstream of the coils of thedownstream evaporator.

FIG. 9 shows a variation of the separate configuration of FIG. 7 wherecooling circuit 302′ includes a suction line heat exchanger 900 having afirst heat exchange path 902 coupled between an outlet 904 of downstreamevaporator 312 and an inlet 906 of tandem digital scroll compressor 718.A second heat exchange path 908 of suction line heat exchanger 900 iscoupled between an outlet of 910 of condenser 316 and an inlet 912 ofexpansion valve 314.

In this variation, the suction line heat exchanger 900 subcools thehigh-pressure refrigerant flowing from condenser 316 through heatexchange path 908 resulting in superheating the gas phase refrigerantflowing through heat exchange path 902 from downstream evaporator 312 totandem digital scroll compressor 718 so that the gas phase refrigerantis superheated when it enters tandem digital scroll compressor 718. Thisfrees downstream evaporator 312 from doing any superheating and achievesa comparable increase in efficiency of tandem digital scroll compressor718 (i.e., increase in evaporating temperature) as the interleavedconfiguration.

In the embodiment of FIG. 9, only downstream cooling circuit 302′includes a suction line heat exchanger. FIG. 10 shows a variation ofFIG. 9 in which cooling circuit 301′ also includes a suction line heatexchanger 1000 having a first heat exchange path 1002 coupled between anoutlet 1004 of upstream evaporator 304 and an inlet 1006 of compressor310. A second heat exchange path 1008 of suction line heat exchanger1000 is coupled between an outlet of 1010 of condenser 308 and an inlet1012 of expansion valve 306.

In this variation, the suction line heat exchanger 1000 subcools thehigh-pressure refrigerant flowing from condenser 308 through heatexchange path 1008 resulting in superheating the gas phase refrigerantflowing through heat exchange path 1002 from upstream evaporator 304 totandem digital scroll compressor 710 so that the gas phase refrigerantis superheated when it enters tandem digital scroll compressor 710. Thisfrees upstream evaporator 304 from doing any superheating and increasesthe efficiency of tandem digital scroll compressor 710 (i.e., increasein evaporating temperature).

A cooling system that has staged cooling such as CRAC 700 (FIG. 7) thatutilizes a plurality of cooling circuits such as cooling circuits 301,302 as discussed above with tandem digital scroll compressors 710, 718allows for better optimization of sensible cooling control and ofdehumidification control.

In an aspect, controller 320 controls the tandem digital scrollcompressors 710, 718. Controller 320 is illustratively programmed withappropriate software that implements the below described control oftandem digital scroll compressors 710, 718. Controller 320 mayillustratively be an iCOM® control system available from LiebertCorporation of Columbus, Ohio programmed with software implementing theadditional functions described below.

As used herein Call for Cooling means the cooling demand which is theactual cooling that the cooling system is being called on to provide.Typically, “Call for Cooling” is expressed as the percentage of theoverall or nominal maximum cooling capacity of the cooling system. Itshould be understood that it can be expressed other than as apercentage. For example, it could be expressed in terms of power, suchas kilowatts (Kw). By way of example only and not of limitation, thecooling system may have an overall capacity of 125 Kw and if it beingcalled on to provide 62.5 Kw of cooling, the Call for Cooling couldexpressed at 62.5 Kw, as well as 50%.

Turning first to control of sensible cooling, controller 320 controlswhich fixed capacity compressor and digital scroll compressor of eachtandem digital scroll compressor are on and in the case of each digitalscroll compressor, its loading, based on the Call for Cooling and whichof a plurality of ranges it falls within. In an aspect, the controllerfirst begins ramping the variable capacity digital scroll compressor ofthe cooling circuit of the upstream cooling stage to operate theupstream cooling stage to provide cooling. When the Call for Coolingincreases to a point where it will be more efficient to operate thedownstream cooling stage to provide additional cooling rather thancontinuing to only increase the ramping of the variable capacity digitalscroll compressor of the cooling circuit of the upstream cooling stage,the controller also begins ramping the variable capacity digital scrollcompressor of the cooling circuit of the downstream cooling stage inparallel with ramping the variable capacity digital scroll compressor ofthe upstream cooling circuit. This operates both the upstream coolingstage and the downstream cooling stage to provide cooling. In so doing,controller 320 balances maximizing the operation of the variablecapacity digital scroll compressor, particularly of the tandem digitalscroll compressor of the cooling circuit of the upstream cooling stage,with the operation of the cooling circuit of the downstream coolingstage to better optimize efficiency.

In the following example, controller 320 has four control modesdetermined by the Call for Cooling (expressed as a percentage in thefollowing example) that the cooling system (such as cooling system 300)is being called to provide, determined by controller 320 when cooling isramping up and also when cooling is ramping down. FIG. 11A is a tablethat shows these control modes for the control of the fixed capacityscroll compressors 710(F) and 718(F) and the digital scroll compressors710(V) and 718(V) of each of tandem digital scroll compressors 710, 718.The term “ramp” when used in the table of FIG. 11 A with respect to adigital scroll compressor means the capacity of the digital scrollcompressor is being modulated up (increasing the percentage of time thatthe digital scroll compressor is loaded) or down (decreasing thepercentage of time that the digital scroll compressor is unloaded) asappropriate to provide fine adjustment of cooling to meet the coolingdemand, referred to as Call for Cooling as discussed above. In table11A, based on where the Call for Cooling compares to sensible coolingramping up control thresholds SRU1-SRU5 (that is, what range the Callfor Cooling falls within) determines when the fixed capacity scrollcompressors 710(F) and 718(F) are on and when digital scroll compressors710(V) and 718(V) are on and their percentage loading when the Call forCooling is ramping up. The values in parentheses next to each controlthreshold SRU1-SRU5 are illustrative preferred values for each of thesecontrol thresholds. It should be understood, however, that controlthresholds SRU1-SRU5 can have different values and these values may bedetermine heuristically and/or theoretically to optimize these values.

Similarly in table 11A, based on where the Call for Cooling percentagecompares to sensible cooling ramping down control thresholds SRD1-SRD5determines when the fixed capacity scroll compressors 710(F) and 718(F)are on and when digital scroll compressors 710(V) and 718(V) are on andtheir percentage loading when the Call for Cooling is ramping down.Again, the values in parentheses next to each control thresholdSRD1-SRD5 are illustrative preferred values for each of these controlthresholds. It should be understood, however, that control thresholdsSRD1-SRD5 can have different values and these values may be determinedheuristically and/theoretically to optimize these values.

It should be understood that the above discussed four control modes areillustrative and there can be other than four control modes,particularly, if there are more than two cooling stages and there thusbeing more than two tandem digital scroll compressors (e.g., a tandemdigital scroll compressor for each cooling stage).

Turning to control of dehumidification, controller 320 controls whichfixed capacity compressor and variable capacity digital scrollcompressor of each tandem digital scroll compressor are on based on theCall for Cooling and which of a plurality of dehumidification controlranges it falls within and then controls ramping of the applicablevariable capacity digital scroll compressor based on a Call forDehumidification. In the following example, controller 320 has threecontrol modes determined by the Call for Cooling. FIG. 11B is a tablethat shows these control modes for the control of the fixed capacityscroll compressors 710(F) and 718(F) and the digital scroll compressors710(V) and 718(V) of each of tandem digital scroll compressors 710, 718.The same terms used in the table of FIG. 9A are also used in the tableof FIG. 9B. In addition, “Call for Dehumidification” means thepercentage of dehumidification for which cooling system 700 is beingcalled to provide, determined by controller 320. In table 11B, based onwhere the Call for Cooling compares to latent cooling control thresholdsL1-L4 determines when the fixed capacity scroll compressors 710(F) and718(F) are on and when digital scroll compressors 710(V) and 718(V) areon and whether they are being ramped. The Call for Dehumidificationdetermines the ramping of each of variable capacity digital scrollcompressor 710(V) and 718(V) that is being ramped. Again, the values inparentheses next to each control threshold L1-L4 are illustrativepreferred values for each of these control threshold that definedehumidification control ranges. It should be understood, however, thatcontrol thresholds L1-L4 can have different values and these values maybe determine heuristically and/theoretically to optimize these values.In an illustrative aspect, the control modes shown in the table of FIG.11B take precedence over the control modes shown in the table of FIG.11A when dehumidification is being called for—when there is an unmetCall for Dehumidification. When the Call for Dehumidification is met,control switches back to the control modes shown in the table of FIG.11A. It should also be understood that the three control modes areillustrative and that three can be other than three control modes,particularly, if there are more than two cooling stages and there thusbeing more than two tandem digital scroll compressors (e.g., a tandemdigital scroll compressor for each cooling stage).

FIG. 11C is a basic flow chart of a software program for controller 320to control tandem digital scroll compressors 710, 718 in accordance withthe control set points set out in Tables 11A and 11B. At 1102,controller 320 determines whether there is a Call for Dehumidificationand if so, the percentage of the Call for Dehumidification. If there wasa Call for Dehumidification, at 1104 controller 320 controls tandemdigital scroll compressors 710, 718 based on the Call for Cooling andcontrol thresholds L1-L4 in table 11B. That is, based on where the Callfor Cooling falls in the range of control thresholds L1-L4, controller320 turns fixed capacity scroll compressors 710(F), 718(F) on and offand turns digital scroll compressors 710(V) and 718(V) on and offwhether to ramp them if on. Based on the Call for Dehumidification,controller 320 controls the ramping of each variable capacity digitalscroll compressor 710(V) and 718(V) that is being ramped. Controller 320then returns to block 1102.

If at 1102 controller 320 determined that there was not a Call forDehumidification, at 1106 it determines whether there was a Call forCooling and the percentage of the Call for Cooling. If not, controller320 returns to block 1102. If controller 320 determined that there was aCall for Cooling, at 1108 controller 320 determines if cooling isramping up. If so, at 1110 controller 320 controls tandem digital scrollcompressors 710, 718 based on the percentage of the Call for Cooling andcontrol thresholds SRU1-SRU5 in the cooling ramping up portion of table11A. That is, based on where the percentage of the Call for Coolingfalls in the range of control thresholds SRU1-SRU5, controller 320 turnsfixed capacity scroll compressors 710(F), 718(F) on and off and alsoturns on and off digital scroll compressors 710(V), 718(V) and setstheir percentage of loading. Controller 320 then returns to block 1102.If at 1108 controller 320 determined that cooling was not ramping up,cooling is ramping down and at 1112 controller 320 controls tandemdigital scroll compressors 710, 718 based on the percentage of the Callfor Cooling and control thresholds SRD1-SRD5 in the cooling ramping downportion of table 11A. That is, based on where the percentage of the Callfor Cooling falls in the range of control thresholds SRD1-SRD5,controller 320 turns fixed capacity scroll compressors 710(F), 718(F) onand off and also turns on and off digital scroll compressors 710(V),718(V) and sets their percentage of loading. Controller 320 then returnsto block 1102.

While the above description of staged cooling was in the context of datacenter cooling system having a CRAC, it should be understood that thestaged cooling can be used in other types of cooling systems, such asbuilding HVAC systems used for comfort cooling, such as cooling offices.

While the downstream evaporator discussed above was a A-coil assembly,and in an aspect the upstream evaporator discussed above was also aA-coil assembly, it should be understood that the staged cooling systemcould utilize a V-coil assembly as the downstream evaporator and in anaspect, utilize an V-coil assembly as the upstream evaporator It shouldalso be understood that the upstream and downstream evaporators couldeach utilize a large, inclined cooling slab, or a flat cooling slab.

In accordance with another aspect of the present disclosure, a coolingsystem, which may include a CRAC, includes a DX cooling circuit with apumped refrigerant economizer enabling the system to be run in a pumpedrefrigerant economizer mode when the temperature outside is cold enoughto cool the cooling fluid circulating in the cooling circuit and bypassthe compressor. The cooling fluid may illustratively be a phase changerefrigerant having a vapor phase and a liquid phase. The pumpedrefrigerant economizer may illustrativley include a pump that circulatesthe cooling fluid, illustratively the refrigerant in its liquid phase,with the compressor bypassed. This cooling system then uses the pumpinstead of the compressor to pump the refrigerant in its liquid phaseand circulate the refrigerant when the outside air temperature is lowenough to provide the heat exchange without compressing the refrigerantin its vapor phase to a higher pressure/condensing temperature. Theeconomizer mode significantly increases the cooling system's sensiblecoefficient of performance (COP) when the cooling system switches to theeconomizer mode as described below. In terms of annual efficiency, theclimate determines the benefit. For instance, modeling has shown thatthe annual energy efficiency increase in Washington DC is about 26%,while in Minneapolis, Minn., the annual energy efficiency increase isabout 53%.

As discussed above, a conventional DX air conditioning system containsan evaporator, a compressor, a condenser and an expansion device. Oftenthe air being cooled is at a lower temperature than the outside air.Because of this, a compressor is required to raise the pressure of therefrigerant in its vapor phase, and therefore its condensingtemperature, to a higher temperature than the outside air so that theheat can be rejected. In any application in which heat is rejected tothe outdoors even in the middle of the winter, the need to compress thecooling fluid consumes energy unnecessarily.

When the outdoor temperature becomes low enough to provide the overallrequired temperature difference between the inside air from which theheat is removed and the outside air to which the heat is rejected, thereis no need to compress the refrigerant in its vapor phase to a higherpressure/temperature. When that is the case, the cooling system inaccordance with this aspect of the present disclosure switches from DX(compressor) mode to pumped refrigerant economizer mode. In the pumpedrefrigerant economizer mode, the refrigerant is pumped in its liquidphase by a liquid pump to circulate the refrigerant in the coolingcircuit without compressing the refrigerant in its vapor phase. Theadvantage is that the pump consumes roughly 1/10 of the power consumedby the compressor.

The temperature at which the controller of the cooling system having apumped refrigerant economizer mode decides to switch from one mode tothe other is based on the difference between the indoor and outdoortemperatures, and the heat load on the cooling system. As stated above,the cooling system described here includes the components listed above,which are the typical components of a DX cooling circuit described withreference to FIG. 2, as well as a pump. When the controller decides toswitch from DX (compressor) mode to pumped refrigerant economizer mode,the compressor is turned off and the pump is turned on. In the pumpedrefrigerant economizer mode, the refrigerant is bypassed around thecompressor, while in DX (compressor) mode, the refrigerant is bypassedaround the pump.

The following description of embodiments of a cooling system having a DXcooling circuit and a pumped refrigerant economizer will show preferredand alternative system layouts and component functionality. The threemain control considerations for this system operating in the pumpedrefrigerant economizer mode are capacity control, evaporator freezeprevention (outdoor temperature can get very low) and pump protection.Most pumps require a minimum differential to ensure adequate cooling ofthe motor (if the pump is a canned motor pump) and lubrication of thebearings. Each of these control functions can be accomplished by a fewdifferent methods using different components.

With reference to FIG. 12, a preferred embodiment of a cooling system1200 having a pumped refrigerant economizer mode is shown. Coolingsystem 1200 includes a DX cooling circuit 1202 having an evaporator1204, expansion valve 1206 (which may preferably be an electronicexpansion valve but may also be a thermostatic expansion valve),condenser 1208 and compressor 1210 arranged in a DX refrigerationcircuit. Cooling circuit 1202 also includes a fluid pump 1212, solenoidvalve 1214 and check valves 1216, 1218, 1222. An outlet 1262 ofcondenser 1208 is coupled to an inlet 1228 of pump 1212 and to an inlet1230 of check valve 1216. An outlet 1232 of pump 1212 is coupled to aninlet 1234 of solenoid valve 1214. An outlet 1236 of solenoid valve 1214is coupled to an inlet 1238 of electronic expansion valve 1206. Anoutlet 1240 of check valve 1216 is also coupled to the inlet 1238 ofelectronic expansion valve 1206. An outlet 1242 of electronic expansionvalve 1206 is coupled to a refrigerant inlet 1244 of evaporator 1204. Arefrigerant outlet 1246 of evaporator 1204 is coupled to an inlet 1248of compressor 1210 and to an inlet 1250 of check valve 1218. An outlet1252 of compressor 1210 is coupled to an inlet 1254 of check valve 1222and an outlet 1256 of check valve 1222 is coupled to an inlet 1258 ofcondenser 1208 as is an outlet 1260 of check valve 1218.

Cooling system 1200 also includes a controller 1220 coupled tocontrolled components of cooling system 1200, such as electronicexpansion valve 1206, compressor 1210, pump 1212, solenoid valve 1014,condenser fan 1224, and evaporator air moving unit 1226. Controller 1220is illustratively programmed with appropriate software that implementsthe below described control of cooling system 1200. Controller 1220 mayinclude, or be coupled to, a user interface 1221. Controller 1220 mayillustratively be an iCOM® control system available from LiebertCorporation of Columbus, Ohio programmed with software implementing theadditional functions described below.

Pump 1212 may illustratively be a variable speed pump but alternativelymay be a fixed speed pump. Condenser fan 1224 may illustratively be avariable speed fan but alternatively may be a fixed speed fan.

Where pump 1212 is a variable speed pump, cooling capacity of coolingcircuit 1202 when in the pumped refrigerant economizer mode iscontrolled by controller 1220 by modulating the speed of pump 1212. Thatis, to increase cooling capacity, controller 1220 increases the speed ofpump 1212 to increase the rate of flow of refrigerant in cooling circuit1202 and to decrease cooling capacity, controller 1220 decreases thespeed of pump 1212 to decrease the rate of flow or refrigerant incooling circuit 1202. The refrigerant temperature at the inlet ofevaporator 1204 is maintained above freezing by controller 1220modulating the speed of fan 1224 of condenser 1208 and the minimum pumpdifferential is maintained by controller 1220 modulating the electronicexpansion valve 1206. Pump differential means the pressure differentialacross the pump. In this regard, when pump 1212 is a variable speedpump, it may illustratively be a hermetically sealed pump cooled by therefrigerant that is flowing through it as it is pumping the refrigerantand thus a minimum pump differential is needed so that pump 1212 isadequately cooled.

Where pump 1212 is a fixed speed pump, cooling capacity of coolingcircuit 1202 is controlled by controller 1220 modulating electronicexpansion valve 1206 to increase or decrease the rate of flow ofrefrigerant in cooling circuit 1202.

In a preferred embodiment, the pump 1212 is in a box that sits outsideby the condenser, but the pump 1212 could also be in the indoor unit insome of the embodiments.

In DX (compressor) mode, controller 1220 controls compressor 1210 to berunning, solenoid valve 1214 to be closed and pump 1212 to be off. Sincecompressor 1210 is running, suction at an inlet 1248 of compressor 1210inlet draws vaporized refrigerant from an outlet 1246 of evaporator 1204into compressor 1210 where it is compressed by compressor 1210, raisingits pressure. The suction at the inlet 1248 of running compressor 1210will draw the refrigerant into the inlet 1248 and it doesn't flowthrough check valve 1218. The refrigerant then flows through check valve1222 into condenser 1208 where it is cooled and condensed to a liquidstate. Since solenoid valve 1214 is closed and pump 1212 is off, afterthe refrigerant flows out of condenser 1208 it flows through check valve1216, through expansion valve 1206 where its pressure is reduced andthen into evaporator 1204. The refrigerant flows through evaporator1204, where it is heated to vaporization by air to be cooled flowingthrough evaporator 1204, and then back to the inlet 1248 of compressor1210.

When controller 1220 switches cooling circuit 1202 to the pumpedrefrigerant economizer mode, it opens solenoid valve 1214, turnscompressor 1210 off and pump 1212 on. Pump 1212 then pumps therefrigerant to circulate it and it flows through solenoid valve 1214,electronic expansion valve 1206, evaporator 1204, check valve 1218bypassing compressor 1210, through condenser 1208 and back to an inlet1228 of pump 1212. Controller 1220 switches cooling circuit 1202 to thepumped refrigerant economizer mode when the temperature of the outsideair is cold enough to provide the requisite temperature differentialbetween the inside air to be cooled and the outside air to which heat isrejected.

In an aspect, an inverted trap 1264 may be coupled between outlet 1236of valve 1214 and inlet 1238 of electronic expansion valve 1206 as shownin phantom in FIG. 12.

In an aspect, a receiver/surge tank, such as receiver surge/tank 1706described below, may be coupled between outlet 1262 of condenser 1208 aninlet 1228 of pump 1212 so that all refrigerant flow through thereceiver/surge tank prior to entering inlet 1228.

FIG. 13 shows a cooling system 1300 having a cooling circuit 1302 thatis a variation of cooling circuit 1202. With the following differences,cooling system 1300 is otherwise essentially the same as cooling system1200 and otherwise operates in the same manner as cooling system 1200.In cooling system 1300, a solenoid valve 1304 is added at the inlet 1248of compressor 1210 that is controlled by controller 1220 to preventliquid slugging to the compressor. When cooling system 1300 is in the DX(compressor) mode, controller 1220 opens solenoid valve 1304. Whencooling system 1300 is in the pump refrigerant economizer mode,controller 1220 closes solenoid valve 1304 thus preventing refrigerantfrom flowing to inlet 1248 of compressor 1210 and preventing liquidslugging of compressor 1210. A bypass solenoid valve 1306 is also addedaround electronic expansion valve 1206 and a distributor (not shown)that distributes the refrigerant to the circuits of the evaporatorincludes an inlet port that bypasses the orifice of the distributor, andthe outlet of the bypass solenoid valve 1306 is plumbed to this bypassinlet to reduce system pressure drop.

FIG. 14 shows a cooling system 1400 that is a variation of the coolingsystem 1200 shown in FIG. 12 having a cooling circuit 1402. With thefollowing differences, cooling system 1400 is otherwise essentially thesame as cooling system 1200 and otherwise operates in the same manner ascooling system 1200. In cooling system 1400, check valve 1216 bypassingthe pump 1212 has been eliminated, also resulting in the elimination ofsolenoid valve 1214. In this case, the refrigerant would flow throughthe pump 1212 when the cooling circuit is in DX (or compressor) mode.This assumes that the pump 1212 is not damaged by passive rotation.

FIG. 15 shows a cooling system 1500 that is a variation of coolingsystem 1200 having a cooling circuit 1502. With the followingdifferences, cooling system 1500 is otherwise essentially the same ascooling system 1200 and otherwise operates in the same manner as coolingsystem 1200. In cooling system 1500, the pump differential is maintainedby controller 1220 modulating a discharge control valve 1504 atdischarge outlet 1506 of pump 1212. It should be understood that whiledischarge control valve 1504 is shown with the same valve symbol as usedfor solenoid valves, discharge control valve 1504 is a variable flowvalve as opposed to an on-off valve. Cooling system 1500 also includesbypass solenoid valve 1304 (FIG. 13) around expansion valve 1206 (whichcould be either an electronic or thermostatic expansion valve) and thedistributor orifice of the distributor (not shown) that distributesrefrigerant to the circuits of the evaporator to reduce system pressuredrop. In this embodiment, pump 1212 is variable speed pump andcontroller 1220 modulates the speed of pump 1212 to control a flow rateof the refrigerant being circulated to control the cooling capacity ofcooling system 1500 when cooling system 1500 is in the pumpedrefrigerant economizer mode.

FIG. 16 illustrates a cooling system 1600 that is a variation of coolingsystem 1500 having a cooling circuit 1602. With the followingdifferences, cooling system 1500 is otherwise essentially the same ascooling system 1500 and otherwise operates in the same manner as coolingsystem 1500. Cooling system 1600 has an alternative method ofmaintaining minimum refrigerant temperature. More specifically, coolingsystem 1600 has a bypass line 1603 around the condenser 1208 with abypass control valve 1604 in bypass line 1603 to allow flow of the warmrefrigerant around the condenser 1208 to mix with cold refrigerantflowing from an outlet 1606 of condenser 1208 to maintain the desiredtemperature and prevent evaporator freezing. Bypass control valve 1604is a variable flow valve and is illustratively controlled by controller1220. A check valve 1608 is coupled between outlet 1262 of condenser1208 and inlet 1228 of pump 1212, with an outlet 1612 of check valve1608 being coupled to inlet 1228 of pump 1212. An outlet 1610 of bypasscontrol valve 1604 is also coupled to inlet 1228 of pump 1212, and thusalso coupled to an outlet 1612 of check valve 1608.

FIG. 17 shows a cooling system 1700 that is a variation of coolingsystem 1600 having a cooling circuit 1702. With the followingdifferences, cooling system 1700 is otherwise essentially the same ascooling system 1600 and otherwise operates in the same manner as coolingsystem 1600. Cooling circuit 1702 of cooling system 1700 also includessolenoid valve 1304 (FIG. 13) at the inlet 1248 of compressor 1210 toprevent liquid slugging to compressor 1210. Because evaporator 1204would be overfed as discussed below, and liquid refrigerant would beleaving the evaporator 1204, solenoid valve 1304 is used to preventliquid slugging to the compressor 1210. Cooling circuit 1702 of coolingsystem 1700 also includes bypass line 1603 around the condenser 1208with bypass control valve 1604 (FIG. 16) and check valve 1608 coupledbetween a pressure regulating valve 1703 and to an inlet 1704 ofreceiver/surge tank 1706 and to inlet 1228 of pump 1212. An outlet 1708of receiver/surge tank 1706 is coupled to inlet 1228 of pump 1212.Outlet 1610 of bypass valve 1604 is also coupled to inlet 1704 ofreceiver/surge tank 1706 and to inlet 1228 of pump 1212. In thepreviously discussed embodiments, no receiver/surge tank 1706 tank isrequired because the cooling system is run in pumped refrigeranteconomizer mode by controller 1220 with the same distribution ofrefrigerant as in DX (compressor) mode (liquid between the condenser andthe evaporator inlet, liquid-vapor mix in the evaporator, and vaporbetween the evaporator outlet and the condenser inlet). Withreceiver/surge tank 1706, controller 1220 can run cooling system 1700 tooverfeed evaporator 1204 so that there would be a liquid-vapor mixbetween evaporator outlet 1246 and condenser 1208. This increases thecooling capacity of cooling system 1700 compared to the previouslydiscussed embodiments, but the addition of receiver/surge tank 1706 addscost. It should be understood that receiver/surge tank 1706 can be usedwith the previously discussed embodiments and dong so makes the systemless charge sensitive. That is, the system can accommodate widervariations in refrigerant charge levels.

FIG. 18 shows a cooling system 1800 that is a variation of coolingsystem 1700 having a cooling circuit 1802. With the followingdifferences, cooling system 1800 is otherwise essentially the same ascooling system 1700 and otherwise operates in the same manner as coolingsystem 1700. Cooling system 1800 has a different function for the bypasscontrol valve 1604 in bypass line 1603. In this case, the condenserbypass line 1603 enters the receiver/surge tank with outlet 1610 ofbypass control valve 1604 coupled to inlet 1704 of receiver/surge tank1706 but not to inlet 1228 of pump 1212. Outlet 1612 of check valve 1608is also not coupled to inlet 1704 of receiver/surge tank 1706 andpressure regulating valve 1703 is eliminated. Controller 1220 modulatesbypass control valve 1604 to modulate the pressure of receiver/surgetank to force liquid from receiver/surge tank 1706 to inlet 1228 of pump1212. This is similar to the method described in U.S. Pat. No.7,900,468, the entire disclosure of which is incorporated herein byreference. Controller 1220 may illustratively be programmed to utilizethe method descried in U.S. Pat. No. 7,900,460.

FIG. 19A shows a cooling system 1900 that is a variation of coolingsystem 1700 having a cooling circuit 1902. With the followingdifferences, cooling system 1900 is otherwise essentially the same ascooling system 1700 and otherwise operates in the same manner as coolingsystem 1700. Outlet 1610 of bypass control valve 1604 is coupled throughcheck valve 1904 to inlet 1704 of receiver/surge tank 1706 and to inlet1228 of pump 1212 and outlet 1612 of check valve 1608 is also coupled toinlet 1704 of receiver/surge tank 1706 and to inlet 1228 of pump 1212.The refrigerant preferentially flows through receiver/surge tank 1706prior to entering inlet 1228 of pump 1212, but may flow around receiversurge tank 1706.

FIG. 19BA shows a cooling system 1900′ that is also a variation ofcooling system 1700 having cooling circuit 1902′. Bypass control valve1604 and check valve 1904 are eliminated and the outlet of check valve1608 is coupled to the inlet 1704 of receiver/surge tank 1706 but not tothe inlet 1228 of pump 1212. In cooling system 1900′, all therefrigerant flows through receiver/surge tank 1706 prior to enteringinlet 1228 of pump 1212.

FIG. 20 shows a cooling system 2000 that is a variation of coolingsystem 1700 having a cooling circuit 2002. With the followingdifferences, cooling system 2000 is otherwise essentially the same ascooling system 1700 and otherwise operates in the same manner as coolingsystem 1700. In cooling system 2000, a three-way valve 2004 is coupledbetween outlet 1246 of evaporator 1204, inlet 1248 of compressor 1210and inlet 1258 of condenser 1208, with solenoid valve 1304 and checkvalve 1218 being eliminated. Controller 1220 controls three-way valve2004 to provide refrigerant to compressor 1210 when in the directexpansion mode and to bypass compressor 1210 when in the pumpedrefrigerant economizer mode.

FIG. 21 shows a cooling system 2100 that is a variation of coolingsystem 1600 having a cooling circuit 2102. With the followingdifferences, cooling system 2100 is otherwise essentially the same ascooling system 1600 and otherwise operates in the same manner as coolingsystem 1600. In cooling system 2100, a suction line accumulator 2104 isdisposed at inlet 1248 of compressor 1210 to prevent liquid slugging tocompressor 1210. Also, cooling system 2100 has solenoid valve 1214instead of discharge control valve 1504, solenoid valve 1214 beingoperated in the manner described for cooling system 1200.

FIG. 22 shows a cooling system 2200 that is a variation of coolingsystem 1600 having a cooling circuit 2202. With the followingdifferences, cooling system 2200 is otherwise essentially the same ascooling system 1600 and otherwise operates in the same manner as coolingsystem 1600. Cooling system 2200 includes a suction line heat exchanger2204 that increases the evaporator charge when cooling system 2200 is inthe direct expansion mode to normalize evaporator charge between thedirect expansion mode and the pumped refrigerant economizer node.Suction line heat exchanger 2204 is bypassed when cooling system 2200 isin the pumped refrigerant economizer mode by controller 1220 openingbypass solenoid valve 1306 (which in the embodiment of FIG. 22 iscoupled around a first exchange path 2206 of suction line heat exchanger2204 as well as electronic expansion valve 1206). It should beunderstood that controller 1220 closes solenoid valve 1306 when coolingsystem 2200 is in the direct expansion mode. First heat exchange path2206 of suction line heat exchanger 2204 is coupled between a junction2208 to which outlet 2210 of check valve 1216 and an outlet 2212 ofsolenoid valve 1214 are coupled and an inlet 2214 of electronicexpansion valve 1206. A second heat exchange path 2216 of suction lineheat exchanger 2204 is coupled between outlet of 1246 of evaporator 1204and a junction 2218 to which inlet 1248 of compressor 1210 and inlet2220 of check valve 1218 are coupled.

FIG. 23 shows a free-cooling system 2300 that is a variation of coolingsystem 1200 having a cooling circuit 2302, which would be forapplications in which the condenser 1208 is at an elevationsignificantly higher than the evaporator 1204. In this case, whencooling system 2300 is in the economizer mode, the liquid column ofrefrigerant at the inlet 1248 of evaporator 1204 induces a thermo-siphoneffect causing refrigerant to flow from outlet 1262 of condenser 1208through electronic expansion valve 1206, through evaporator 1204,through check valve 1218 and back to condenser 1208. Cooling capacity ofcooling system 2300 is controlled by controller 1220 modulating theelectronic expansion valve 1206. The advantage of this system is thatthe only additional component required to be added to a conventional DXsystem is the compressor bypass valve, which in the embodiment shown inFIG. 23, is check valve 1218. While the advantage of cooling system 2300is that no pump 1212 (and associated solenoid valve 1214 and check valve1218 is required, the same method of control used for cooling system2300 can also be used for any of the previously discussed coolingsystems 1200-2200 as long as the condenser 1208 is high enough above theevaporator 1204. Since the flow rate of refrigerant induced by thethermo-siphon effect increases with increasing height of the liquidcolumn of refrigerant, the pump 1212 could be turned off when the loadrequirement is low enough for the given application, and more energycould be saved.

FIG. 24 shows a cooling system 2400 that is another alternativeembodiment having a cooling circuit 2402, in which a free-cooling systemsuch as free-cooling system 2300 is combined with a known liquidoverfeed system of the type where inlet 1246 of evaporator 1204 isoverfed when cooling system 2400 is in the direct expansion mode suchthat the refrigerant enters evaporator 1204 as a slightly subcooledliquid instead of a two phase mixture, making for more evendistribution. In cooling system 2400, outlet 1262 of condenser 1208 iscoupled through electronic expansion valve 1206 to an inlet 2403 ofliquid/vapor separator tank 2404. A vapor outlet 2406 of liquid/vaporseparator tank 2404 is coupled to inlet 1248 of compressor 1210. Aliquid outlet 2408 of liquid/vapor separator tank 2404 is coupled toinlet 1228 of pump 1212. An outlet 2410 of compressor 1210 is coupled toinlet 1254 of check valve 1222. Outlet 1256 of check valve 1222 iscoupled to an inlet 1258 of condenser 1208. An inlet 2422 of solenoidvalve 2420 and an inlet 2424 of solenoid valve 2424 are coupled tooutlet 1246 of evaporator 1204. An outlet 2418 of solenoid valve 2420 iscoupled to inlet 1258 of condenser 1208. An outlet 2428 of solenoidvalve 2426 is coupled to a second inlet 2430 of liquid/vapor separatortank 2404. Controller 1220 controls pump 1212 so that pump 1212 isalways on, either pumping (circulating) the refrigerant throughevaporator 1204 and back to liquid/vapor separator tank 2404, or pumpingthe refrigerant through evaporator 1204 with the compressor 1210bypassed and turned off by controller 1220 to save energy. In thisregard, controller 1220 controls solenoid valves 2420 and 2426 asdiscussed below.

Depending on the type of evaporator used, even distribution of thetwo-phase refrigerant at the inlet of the evaporator is difficult tomaintain in a conventional DX system in which the refrigerant fluid isexpanded upstream of the evaporator. This is particularly the case withmicrochannel heat exchangers. Cooling system 2400 includes a liquidoverfeed system having pump 1212 that provides liquid refrigerant toinlet 1244 of evaporator 1204, mitigating the distribution issues. Therefrigerant is then evaporated in evaporator 1204 and circulated as atwo-phase mixture back to liquid/vapor separator tank 2404. Thecompressor 1210 pulls vapor from the liquid/vapor separator tank 2404via vapor outlet 2406 of liquid/vapor separator tank 2404, compresses itto its condensing pressure/temperature, moves it to the condenser 1208where it is condensed and then returned to the liquid/vapor separatortank 2404 as a liquid. The pump 1212 pulls liquid refrigerant from theliquid/vapor separator tank 2404 via liquid outlet 2408 of liquid/vaporseparator tank 2404. The liquid level is maintained in the tank via afloat controlled electronic expansion valve 1206. In this regard, floatcontrolled electronic expansion valve 1206 has a control input 2432coupled to a control output 2434 of a float 2436 in liquid/vaporseparator tank 2404. Control output 2434 of float 2436 mayillustratively provide a modulated control signal to electronicexpansion valve or an on/off control signal to electronic expansionvalve 1206. It should be understood that a float controlled mechanicalexpansion valve could alternatively be used instead of electronicexpansion valve 1206.

The path of the refrigerant would be determined by the solenoid valves2420, 2426. In warm weather, controller 1220 would operate coolingsystem 2400 as described above, controlling solenoid valve 2426 betweenoutlet 1246 of evaporator 1204 and the liquid/vapor separator tank 2404to be open and solenoid valve 2420 to be closed. In cold weather,controller 1220 would turn compressor 1210 off, open solenoid valve 2420and close solenoid valve 2426. Cooling system 2400 would then operate ina pumped refrigerant economizer mode such as described above such aswith reference to FIG. 12.

Cooling system 2400 would become advantageous if the price of coppermakes an aluminum microchannel heat exchanger more cost-effective than acopper tube and fin heat exchanger. In that case, the ability to feedliquid refrigerant to the evaporator inlet would increase systemperformance and efficiency. And if a liquid overfeed system is required,it would be fairly straightforward (from a component standpoint) toallow the system to operate in pumped refrigerant economizer mode suchas in the winter, since only the addition of the compressor bypass valvewould be required.

FIG. 25 shows a cooling system 2500 having a separate pumped refrigeranteconomizer cooling circuit 2502 and a conventional DX cooling circuit301 as described above with reference to FIG. 3. Pumped refrigeranteconomizer cooling circuit includes an evaporator 304, expansion valve306 which may illustratively be an electronic expansion valve, fluidpump 1212 (FIG. 12), and condenser 1208 arranged in a pumped refrigerantcooling circuit such as is disclosed in U.S. patent application Ser. No.10/904,889 the entire disclosure of which is incorporated herein byreference. Controller 320 controls cooling system 2500 so that pumpedrefrigerant economizer cooling circuit is only run when the outsideambient temperature is sufficiently cold that pumped refrigerant coolingcircuit 2502 can provide sufficient cooling to meet the cooling demands,such as of a data center. While evaporator 304 of pumped refrigeranteconomizer cooling circuit 2502 is shown in FIG. 25 as being upstream ofevaporator 304 of DX cooling circuit 301, it should be understood thatevaporator 304 of pumped refrigerant economizer cooling circuit 2502could be downstream of evaporator 304 of DX cooling circuit 301.

The discussions of the cooling circuits of FIGS. 12-24 were based on aone circuit cooling system, or on a two circuit system in which theevaporators are parallel in the air-stream. The cooling circuits ofFIGS. 12-24 can also be utilized for staged cooling as described above,particularly with reference to FIG. 3, where the evaporators of the twocircuits are staged in series in the air stream of air to be cooled.Because of this, the entering air temperature is higher on the upstreamcircuit than on the downstream circuit. Subsequently, the evaporatingtemperature is higher on the upstream circuit as well. So with thestaged system, the upstream circuit will be able to switch over topumped refrigerant economizer before the second, which could still beoperating in DX (compressor) mode depending on the load. For example,two cooling circuits 1202 could be arranged with their evaporators inseries to provide staged cooling. FIG. 26 shows a cooling system 2600having two cooling circuits 1202 arranged to provide staged coolingalong the lines discussed above with regard to FIG. 3. In thisembodiment, compressor 1210 in each of the two cooling circuits 1202 mayillustratively be a tandem digital scroll compressor and controlled asdiscussed above with reference to FIGS. 7-11 A and 11B.

In a staged cooling system having two or more staged cooling circuits,at least the most upstream cooling circuit is a variable capacitycooling circuit and preferably the downstream cooling circuit (orcircuits) is also variable capacity cooling circuits. Such variablecapacity may be provided by the use of a tandem digital scrollcompressor as discussed above. It can also be provided by the use of asingle variable capacity compressor, such as a digital scrollcompressor, a plurality of fixed capacity compressors, or othercombinations of fixed and variable capacity compressors. Variablecapacity is also provided by the liquid pump when the cooling circuit isa pumped refrigerant cooling circuit, or operating in the pumpedrefrigerant economizer mode such as cooling circuit 1202 operating inthe pumped refrigerant economizer mode. The cooling system is controlledbased on the Call for Cooling to stage the operation of the upstream anddownstream cooling circuits to enhance efficiency, as described belowwith reference to staged cooling system 2600 as an example stagedcooling system and the flow chart of FIG. 30.

With reference to FIG. 30, as the Call for Cooling is ramping up itreaches a point at 3000 where cooling is needed. At 3002 controller 1220operates the cooling circuit 1202 that is the upstream cooling circuitto provide cooling. As the Call for Cooling continues to ramp it,controller 1220 operates the cooling circuit 1202 to provide increasedcapacity to meet the Call for Cooling. Eventually the Call for Coolingreaches a point at 3004 where it is more efficient to add a coolingcircuit 1202 that is a downstream cooling circuit to provide additionalcooling capacity rather than only increase the capacity of the coolingcircuit 1202 that is the upstream cooling circuit. This point is beforethe cooling circuit 1202 that is the upstream cooling circuit reachesits maximum capacity. At 3006, controller 1220 operates a coolingcircuit 1202 that is a downstream cooling circuit to provide additionalcooling and balances the operation of the cooling circuits 1202 that arebeing operated to provide cooling to optimize efficiency. The priordescription of the control of tandem digital scroll compressors 710, 718is one example of such control.

The advantage to using a cooling system with staged cooling as discussedabove with this pumped refrigerant economizer is that hours of operationcan be gained in pumped refrigerant economizer mode on the upstreamcooling circuit since it is operating at a higher evaporatingtemperature than either cooling circuit would be in a typical prior artparallel evaporator system. So, energy can be saved for more hours ofthe year. The colder the climate is, the more annual energy efficiencyincrease can be realized.

As has been discussed, in a typical vapor compression refrigerationsystem, a large percentage of system power is used to compress therefrigerant vapor leaving the evaporator, thereby increasing thecondensing temperature of the refrigerant to allow for heat rejection inthe condenser. As described above, particularly with reference to FIG.12, in an aspect of the present disclosure in order to save energy in avapor compression refrigeration system, a pump can be used to moverefrigerant from the condenser to the evaporator when outdoortemperatures are low enough to provide “free” cooling without the needto compress the refrigerant vapor. Such a pumped refrigerant(economizer) system is a precision cooling system with aims of energysavings, high efficiency and optimized system performance. Systemcontrol is important to achieving these objectives. More specifically,the control objectives are divided into three levels with differentpriorities, namely:

-   -   1. Component Safety Level: to guarantee key component safety        -   i) Pump cavitation prevention—Subcooling monitoring        -   ii) Ensuring pump cooling and lubrication        -   iii) Evaporator coil freeze protection    -   2. Performance Level: to run the system functionally and        flawlessly        -   i) Maintain controlled air temperature to the setpoint        -   ii) Proper and smooth working mode switchover        -   iii) Fault detection and alarm handling    -   3. Optimization Level        -   i) Extending economizer running hours        -   ii) Advanced fault detection and diagnosis

The resources available for the system to achieve the above-listedobjectives are the installed actuators, which include a variable-speedpump (e.g., pump 1212 in FIG. 12), a variable-speed condenser fan (e.g.,fan 1224 in FIG. 12) and an electronic expansion valve (EEV) (e.g., EEV1206 in FIG. 12). The first step of the control design is to work out acontrol strategy to decide how to allocate the resources to differentcontrol tasks. In other words, given that the entire economizer systemis a multi-input multi-output system (with multiple actuators andmultiple variables to be controlled), how to decouple the system anddetermine the input-output relationship is the solution that thefollowing control strategy implements. This control strategy issummarized on a high level basis as follows:

-   -   Manipulate the condenser fan to control the refrigerant        temperature leaving the condenser;    -   Manipulate the pump to control system capacity, and ultimately        the air temperature in the controlled space;    -   Manipulate the EEV to control pressure differential across the        pump.

The multi-input and multi-output pumped refrigerant economizer system iscontrolled in a relatively simple way. The system is decoupled intothree feedback control loops which regulate their controlled variablesby manipulating their corresponding control inputs as follows:

The aforementioned control strategy benefits the system in several ways:

-   -   1. The condenser fan controls the refrigerant temperature to a        setpoint such that”        -   a. Refrigerant temperature will not be low enough to freeze            the evaporator coil;        -   b. Subcooling is maximized to prevent pump cavitation;        -   c. Condenser fan speed is optimized to save energy in the            sense that the fan speed can't be further reduced without            compromising subcooling and cooling capacity.    -   2. The pump speed controls refrigerant flow rate, and the        capacity in turn, by controlling the room's air temperature to        the user given setpoint.        -   a. Pump speed is roughly linear with respect to capacity for            a fixed refrigerant temperature, which is maintained by the            condenser fan speed control.        -   b. Linearity facilitates high control precision of the air            temperature in the controlled space.    -   3. The EEV controls the differential pressure across the pump        such that        -   a. The pump motor is sufficiently cooled;        -   b. The pump bearings are sufficiently lubricated.

The entire system energy consumption is optimized by the foregoingcontrol strategy in the sense that no further energy consumption can berealized without sacrificing cooling performance.

FIG. 27 is a schematic of a cooling system 2700 having one coolingcircuit 2702 having a DX cooling circuit 2704 and a pumped refrigeranteconomizer 2706. Cooling system 2700 may physically consist of threeunits: an indoor unit 2708 (illustratively a computer room airconditioner), a pumped refrigerant economizer unit 2710, and anair-cooled condenser unit 2712. The indoor unit 2708 is located insidethe room to be cooled, such as a data center room, and contains themajor components of the DX cooling circuit (other than the condenser1208), including the evaporator 1204, compressor 1210, and expansionvalve 1206, etc. The indoor unit's 2708 functionality is to operate thesystem in a standard direct expansion mode, and also drive the valvesneeded to run the system in pumped refrigerant economizer mode. Thepumped refrigerant economizer unit 2710 is located outside the room andcontains the major components including pump 1212, etc. The pumpedrefrigerant economizer unit 2710 uses liquid pump 1212 to moverefrigerant from the condenser 1208 to the evaporator 1204 when theoutdoor temperatures are low enough to provide “free” cooling withoutrunning a direct expansion refrigeration system. The condenser unit 2712is also located outside the room to be cooled but separated from thepumped refrigerant economizer unit 2710. It cooperates with one of theother two units 2708, 2710 according to heat rejection demand. In FIG.27, “T” in a circle is a temperature sensor and “P” in a circle is apressure sensor, in each instance that are coupled to controller 1220,such as to a respective one of controller boards 2718, 2720, 2722 (whichare discussed below). The temperature sensors include an outside ambientair temperature sensor (shown adjacent condenser 1208) and a supply air(or room return air) temperature sensor (shown adjacent evaporator1204). The remaining temperature sensors sense temperatures of therefrigerant at the indicated locations of cooling circuit 1202 and thepressure sensors sense the pressures of the refrigerant at the indicatedlocations of cooling circuit 1202.

When the cooling system 2700 operates in pumped refrigerant economizermode, there are three feedback control loops for the basic control ofthe pumped refrigerant economizer mode, as shown in FIG. 28.

-   -   A refrigerant temperature feedback control loop 2800 controls        the refrigerant temperature to a setpoint by regulating the        condenser fan speed. The refrigerant temperature is measured at        the pump outlet or at the condenser outlet. In an aspect, the        setpoint is set in the range of 37° F. to 42° F. It should be        understood that these values are exemplar and the fixed setpoint        can be other than 37° F. to 42° F. It should also be understood        that the setpoint can be inputted manually, such as by a user,        or determined by a controller such as controller 1220.    -   A room air temperature feedback control loop 2802 controls the        room's air temperature to the setpoint entered by a user, such        as into controller 1220, by regulating the pump speed.    -   An liquid pump differential pressure feedback control loop 2804        maintains the liquid pump differential pressure (PSID) within a        given range by regulating the EEV 1206 opening. In an aspect,        the given range is set to be 20 PSID to 25 PSID. The given range        is determined by its upper and lower setpoints. It should be        understood that these values are exemplar and the given range        can be other than 20 PSID to 25 PSID. It should also be        understood that that the given range could be input by a user.

Each control loop 2800, 2802, 2804 may illustratively be a processcontrol type of control loop, and may preferably be a PID loop. In theembodiment shown in FIG. 28, each control loop 2800, 2802, 2804 is shownimplemented with a separate controller 2806, 2808, 2810, respectively,such as to co-locate a respective controller board(s) 2718, 2720, 2722(FIG. 27) having each controller 2806, 2808, 2810 in proximity to thedevice it is controlling, and controllers 2806, 2808, 2810 communicatewith each other, such as via a controller area network (CAN) bus. Forexample, the controller board(s) 2718 having controller 2806 is locatedin proximity to condenser 1208 in that controller 2806 controls thespeed of condenser fan 1224. The controller board 2720 having controller2808 is located in proximity to pump 1212 in that controller 2808controls the speed of pump 1212. The controller board(s) 2722 havingcontroller 2810 is collocated in proximity to EEV 1206 in thatcontroller 2810 controls the position of EEV 1206. While controllers2806, 2808, 2810 are implemented on separately located controller boardsin this embodiment, controllers 2806, 2808 and 2810 are collectivelyconsidered part of controller 1220. It should be understood that controlloops 2800, 2802 and 2804 could be implemented on a controller board(s)at a single location along with the remainder of the control functionsof controller 1220.

Refrigerant temperature feedback control loop 2800 has an output atwhich a condenser fan speed control signal is output and has as inputsthe refrigerant temperature setpoint and a feedback signal which is theactual refrigerant temperature, such as by way of example and not oflimitation, at the outlet of the condenser. The room air temperaturefeedback control loop 2802 has an output at which a liquid pump speedcontrol signal is output and has as inputs the room air temperaturesetpoint and a feedback signal which is the actual room air temperature,such as by way of example and not of limitation, at the return air inletof the cooling system. The liquid pump pressure differential controlfeedback loop 2804 has an output at which an electronic expansion valveposition signal is output and having as inputs the given range and afeedback signal which is a pressure differential across the liquid pump.

In order to further improve the transient performance of the refrigeranttemperature control (which is controlled by controlling the speed ofcondenser fan 1224 by control loop 2800), a feedforward controller(controller 2800-1 in FIG. 28) is applied to stabilize refrigeranttemperature by using the pump speed control signal 2803 from controller2808 and the EEV control signal 2805 from controller 2810 as its inputs.The rationale is that refrigerant temperature is related to the flowrate that can be estimated by the pump speed and EEV opening. Theoutputs of controllers 2808 and 2810 of FIG. 28 are fed forward to thecondenser fan speed control loop 2800. The condenser fan speed signalconsists of two parts: feedback signal and feedforward signal. Thus, thecondenser fan can respond by being driven by the feedforward signal inadvance of the feedback signal coming back.

The three control loops have different magnitudes of response time,which prevents the situation in which multiple control elements caninteract to create instability in the control.

This control strategy applies to the pumped refrigerant economizersystem particularly and can also be applied to the class of cooling orair conditioning systems with pumped refrigerant circulation.

The foregoing description of cooling system 2700 is based on a coolingsystem having one cooling circuit. A similar control strategy can beapplied to cooling systems having two cooling circuits, such as thosearranged to provide staged cooling as discussed above. For a coolingsystem having two cooling circuits, such as having staged cooling withtwo cooling circuits, the condenser fan and EEV in the second circuitperform the same respective control tasks as in the first circuit. Thecooling capacity is controlled by the aggregate pump speeds. A controlalgorithm, an example of which is discussed below, determines thecapacity contributed by each pump, and hence decides each pump's speed,as

As discussed, when the cooling system is in the pumped refrigeranteconomizer mode, there are three main controlled parameters: roomtemperature, refrigerant temperature and pump pressure differential(outlet pressure minus inlet pressure). The room temperature iscontrolled by modulating the pump speed via a variable frequency drive.In a cooling system having staged cooling with two or more coolingcircuits, when the cooling system is in the pumped refrigeranteconomizer mode, the cooling load requirement will determine if the pumpin one or more than one of the cooling circuits needs to be operated.

In an illustrative embodiment, a pump startup routine calls foroperating the pump at successively higher speeds until refrigerant flowis established. At each speed, controller 1220 checks to see whetherrefrigerant flow has been established, determined by pump differentialpressure being at least a minimum. If so, the speed of the pump ischanged from the startup speed to the control speed, as described above.If not, controller 1220 turns the pump off for a period of time and thenoperates the pump at the next higher speed.

In an aspect, in the case of a switchover of a cooling circuit fromdirect expansion mode to pumped refrigerant economizer mode, the pump ofthat cooing circuit will be given an initial speed based on the call forcooling at the time of switchover and will go to its initial speed afterthe startup routine is completed. In cooling systems having stagedcooling with a plurality of cooling circuits, this will mean thatdepending on the load, the pumps of more than one cooling circuit maystart immediately at switchover.

The pump pressure differential needs to be maintained above a minimum inorder for cooling and lubricating flow to be provided to the pump motorand bearings. The pump pressure differential for each of pump 1212(upstream) and pump 1212 (downstream) is controlled by position of EEV1206 of the respective cooling circuit 1202 (upstream) and coolingcircuit 1202 (downstream). When controller 1220 switches any of thecooling circuits to pumped refrigerant economizer mode operation, itchanges its control of EEV 1206 of that cooling circuit 1202 fromsuperheat control to manual control, at which time controller 1220provides a signal to that EEV 1206 to control its position based on pumppressure differential.

In an illustrative embodiment, controller 1220 switches the coolingsystem, such as cooling system 2600, to the pumped refrigeranteconomizer mode when there is either a minimum difference between theroom return air temperature entering the cooling system and the outdoorair temperature or the outdoor air temperature is below a minimum. In anaspect, the lower of the actual room return air temperature and thesetpoint is used for the comparison. In an aspect, the minimumtemperature difference between the room return air is 45° F. and theminimum outside air temperature is 35° F. It should be understood thatthese temperatures are examples and minimum temperature difference otherthan 45° F. and a minimum outside air temperature other than 35° F. canbe used. As discussed above, in an aspect, the cooling circuits in asystem having staged cooling may be controlled separately in which casethe room air temperature used for the comparison for each coolingcircuit may be the actual room return air temperature (or its setpointif lower) entering the evaporator 1204 of that cooling circuit 1202.

In an aspect, controller 1220 will switch the cooling system from pumpedrefrigerant economizer mode to direct expansion mode when the pumpedrefrigerant economizer mode is not keeping up with the cooling demand.In the event that the cooling system has staged cooling, in an aspectcontroller 1220 will first switch the most downstream cooling circuitfrom the pumped refrigerant economizer mode to direct expansion mode andif this fails to provide sufficient cooling, then successively switcheseach next upstream cooling circuit in turn to the direct expansion mode.

In an aspect, controller 1220 also switches each cooling circuit fromthe pumped refrigerant economizer mode to the direct expansion modeshould the pump differential pressure of the pump 1212 of that coolingcircuit fall below a predetermined minimum for a predetermined period oftime. This prevents pump failure due to insufficient pump differentialpressure.

In an aspect, controller 1220 also switches each cooling circuit fromthe pumped refrigerant economizer rode to the direct expansion mode ifthe temperature of the refrigerant leaving the pump of that coolingcircuit falls below a predetermined temperature for a predeterminedperiod of time.

In an aspect, controller 1220 may also switch each cooling circuit fromthe pumped refrigerant economizer mode to the direct expansion mode inthe event of a condition indicating a failure of the pumped refrigeranteconomizer mode, such as loss of power to the pump.

Traditional thermostatic expansion valves (TXVs) are used to regulaterefrigerant flow to control evaporator superheat for direct-expansionrefrigeration and air conditioning units that may experience varyingloads. TXVs are mechanically actuated purely by pressure differenceswithin the apparatus. Therefore, a TXV does not directly interact withthe control scheme being used to regulate compressor capacity, whichmeans that the TXV can only behave reactively to adjustments incompressor capacity. In a system which uses pulse width modulation (PWM)to control compressor capacity by unloading, the persistentinterruptions of the compressor mass flow can vary the suction pressure,which introduces the potential for unstable TXV behavior, and poorsuperheat control.

As discussed above, in various aspects of the present disclosures, theexpansion devices used in the cooling circuits are expansion valves andpreferably electronic expansion valves (“EEV”). It should be understood,however, that expansions devices other than expansion valves can beused, such as capillary tubes.

An EEV offers two primary advantages over a TXV: it allows operation atreduced condensing pressure, which contributes to higher system energyefficiency, and it uses programmed logic to move the valve to controlsuperheat. Further, while a tandem digital scroll compressor offers awide range of capacities, it also makes for difficult control ofsuperheat. Because of this, a control strategy for the EEV(s) thatdirectly interacts with the compressor control strategy to provide moreproper and predictable movement of the valve of the EEV, and also allowvalve position monitoring and adjustment by end users.

An illustrative control strategy for the EEV 1206 of each coolingcircuit chooses the most appropriate EEV superheat control mode bycomparing the current operating status of both of the compressors (fixedcapacity compressor and variable capacity digital scroll compressor) inthe tandem digital compressor to a group of related parameters which arelocated in controller 1220 and illustratively set by a user. Thiscontributes to increased predictability, flexibility, and to anincreased level of specialization specific to digital (PWM) scrollcompressor capacity control which cannot be attained with traditionalapplications of TXVs or with EEVs which employ standard control logic.

The illustrative EEV control strategy, illustratively implemented incontroller 1220 such as in software, uses three types of superheatcontrol: Gated, System Mapping, and Constant. A summary of each of thesemodes is provided below.

-   -   1. Gated Mode: Each time the variable capacity digital scroll        compressor unloads, controller 1220 drives the EEV 1206 from its        current position down to minimum open position. Once the        variable capacity digital scroll compressor stops unloading,        controller 1220 opens the EEV is allowed to open to the        position(s) needed to control superheat until the variable        capacity digital scroll compressor unloads again. The minimum        open position may be ten percent open, but it should be        understood that it can be other than ten percent.    -   2. System Mapping Mode: Each time the variable capacity digital        scroll compressor unloads, controller 1220 maintains the EEV        1206 in its current position until the digital scroll compressor        stops unloading. Once unloading has stopped, the EEV is then        free to change its position to achieve control of the superheat        until the variable capacity digital scroll compressor unloads        again.    -   3. Constant Mode: Controller 1220 constantly determines the        superheat and changes the position of EEV 1206 to achieve        superheat control, regardless of the activity of the variable        capacity digital scroll compressor. Constant mode is the default        mode.

The following parameters added to a typical user interface dictate whichof the three superheat control modes are used for different compressoroperation conditions. It should be understood that the values for theparameters discussed below are exemplar and that the parameters can haveother values.

-   -   Parameter A) “Superheat Mode Threshold” [Min: 19%, Def: 80%,        Max: 80%]: Represents the variable capacity digital scroll        compressor PWM loading % at which the superheat control mode may        transition when only the variable capacity digital scroll        compressor in the tandem digital scroll compressor is on.    -   Parameter B) “Digital+Fixed Superheat Threshold” [Min: 19%, Def:        19% Max: 100%]: Represents the variable capacity digital scroll        compressor PWM loading percentage at which the superheat control        mode may transition when both the variable capacity digital        scroll compressor and fixed capacity compressor of the tandem        digital compressor are on.    -   Parameter C) “Digital+Fixed Superheat Control” [System Mapping        OR Constant]: This parameter is used to choose an overall        superheat control mode when both the variable capacity digital        scroll compressor and fixed capacity compressor of the tandem        digital compressor are on.

The following instances of logic are then applied in a software programof a controller, such as controller 1220, to compare the values ofParameters A, B, and C with the current operating status of the tandemdigital scroll compressor to determine which superheat control modeshould be used, also as shown in the flow chart of FIG. 29 showing thelogic for the software program implemented in the controller such ascontroller 1220.

-   -   When only the fixed capacity compressor of the tandem digital        compressor is on:        -   The EEV will operate in Constant Mode for superheat control.    -   When only the variable capacity digital scroll compressor of the        tandem digital compressor is on:        -   If the variable capacity digital scroll compressor's PWM            loading percentage is less than what is set for the            “Superheat Mode Threshold” Parameter “A,” the EEV will            operate in Gated Mode for superheat control.        -   If the variable capacity digital scroll compressor's PWM            loading percentage is greater than or equal to what is set            for the “Superheat Mode Threshold” Parameter “A,” the EEV            will operate in System Mapping Mode for superheat control.    -   When both the variable capacity digital scroll compressor and        fixed capacity compressors of the tandem digital compressor is        on:        -   If the “Digital+Fixed Superheat Control” Parameter “C” is            set to “Constant”, the EEV will operate in Constant Mode for            superheat control.        -   If the “Digital+Fixed Superheat Control” Parameter “C” is            set to “System Mapping”, AND the variable capacity digital            scroll compressor's PWM loading percentage is less than what            is set for the “Digital+Fixed Superheat Threshold” parameter            “B,” the EEV will operate in Gated Mode for superheat            control.        -   If the “Digital+Fixed Superheat Control” Parameter “C” is            set to “System Mapping”, AND the variable capacity digital            scroll compressor's PWM loading percentage is greater than            or equal to what is set for the “Digital+Fixed Superheat            Threshold” Parameter “B,” the EEV will operate in System            Mapping mode for superheat control.

The foregoing EEV control strategy provides the following advantages:more stable control of evaporator superheat while the variable capacitydigital scroll compressor loads and unloads; proper control of superheatmaintains compressor energy efficiency, and reduction of the chance ofdamage to the compressor(s) from liquid refrigerant flood-back. Further,the physical mechanism of the EEV itself allows for reductions in thecondensing pressure of the refrigerant, which increases the tandemdigital compressor's energy efficiency. Also, a user can adjust the EEVsuperheat mode selection and transition points by via a user interface(not shown) to controller 1220.

Spatially relative terms, such as “inner,” “outer,” “beneath”, “below”,“lower”, “above”, “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

As used herein, the term controller, control module, control system, orthe like may refer to, be part of, or include an Application SpecificIntegrated Circuit (ASIC); an electronic circuit; a combinational logiccircuit; a field programmable gate array (FPGA); a processor (shared,dedicated, or group) that executes code; a programmable logiccontroller, programmable control system such as a processor basedcontrol system including a computer based control system, a processcontroller such as a PID controller, or other suitable hardwarecomponents that provide the described functionality or provide the abovefunctionality when programmed with software as described herein; or acombination of some or all of the above, such as in a system-on-chip.The term module may include memory (shared, dedicated, or group) thatstores code executed by the processor.

The term software, as used above, may refer to computer programs,routines, functions, classes, and/or objects and may include firmware,and/or microcode.

The apparatuses and methods described herein may be implemented bysoftware in one or more computer programs executed by one or moreprocessors of one or more controllers. The computer programs includeprocessor-executable instructions that are stored on a non-transitorytangible computer readable medium. The computer programs may alsoinclude stored data. Non-limiting examples of the non-transitorytangible computer readable medium are nonvolatile memory, magneticstorage, and optical storage.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the invention. Individual elements or features ofa particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the invention, and all such modificationsare intended to be included within the scope of the invention.

What is claimed is:
 1. A cooling system, comprising: a cabinet having anair inlet and an air outlet; an air moving unit disposed in the cabinet;a plurality of separate cooling stages including an upstream coolingstage and a downstream cooling stage, the upstream cooling stage havingan upstream cooling circuit and the downstream cooling stage having adownstream cooling circuit; the upstream and downstream cooling circuitsare each a direct expansion refrigeration cooling circuit including anevaporator, a condenser, a tandem compressor and an expansion device,the evaporator having an inlet coupled to an outlet of the expansiondevice and an outlet coupled to an inlet of the tandem compressor, thetandem compressor having an outlet coupled to an inlet of the condenserand the condenser having an outlet coupled to an inlet of the expansiondevice; each tandem compressor including a fixed capacity compressor andvariable capacity compressor; the evaporator of the upstream coolingcircuit (upstream evaporator) and the evaporator of the downstreamcooling circuit (downstream evaporator) arranged in the cabinet so thatair to be cooled passes over them in serial fashion, first over theupstream evaporator and then over the downstream evaporator; and acontroller coupled to the tandem compressors that controls the fixedcapacity compressor and variable capacity compressor of each of thetandem compressors based on a Call for Cooling, which of a plurality ofranges that the Call for Cooling falls within and whether the Call forCooling is ramping up or ramping down.
 2. The cooling system of claim 1wherein the controller first begins ramping the variable capacitycompressor of the upstream cooling circuit to provide cooling and whenthe Call for Cooling increases above a threshold, the controller alsobegins ramping the variable capacity compressor of the cooling circuitof the downstream cooling stage in parallel with ramping the variablecapacity compressor of the upstream cooling circuit.
 3. The coolingsystem of claim 1 wherein the plurality of ranges are defined by thecontroller having Call for Cooling ramping up control thresholds andCall for Cooling ramping down control thresholds, the Call for Coolingramping up control thresholds including SRU1, SRU2, SRU3 and SRU4 thatprogress from lower to higher values, the Call for Cooling ramping downcontrol thresholds including SRD1,SRD2, SRD3 and SRD4 that progress fromlower to higher values; the controller when the Call for Cooling isramping up: when the Call for Cooling is between SRU1 and SRU2, turningon only the variable capacity compressor of the tandem compressor of theupstream cooling circuit and ramping the capacity of that variablecapacity compressor based on the Call for Cooling, when the Call forCooling is between SRU2 and SRU3, turning on only the variable capacitycompressors of the tandem compressors of the upstream cooling circuitand the downstream cooling circuit and ramping their capacity based onthe Call for Cooling, when the call for Call for Cooling is between SRU3and SRU4, turning on the fixed and variable capacity compressors of thetandem compressor of the upstream cooling circuit and ramping thecapacity of that variable capacity compressor based on the Call forCooling and turning on only the variable capacity compressor of thetandem compressor of the downstream cooling circuit and running that thevariable capacity compressor at full capacity, and when the Call forCooling is greater than SRU4 turning on the fixed and variablecompressors of the tandem compressor of the upstream cooling circuit andof the tandem compressor of the downstream cooling circuit, running thevariable capacity compressor of the tandem compressor of the upstreamcooling circuit at full capacity and ramping the capacity of thevariable capacity compressor of the downstream cooling circuit based onthe Call for Cooling; and the controller when the Call for Cooling isramping down: when the Call for Cooling is between SRD1 and SRD2,turning on only the variable capacity compressor of the tandemcompressor of the upstream cooling circuit and ramping the capacity ofthat variable compressor capacity based on the Call for Cooling, whenthe Call for Cooling is between SRD2 and SRD3, turning on only thevariable capacity compressors of the tandem compressors of the upstreamcooling circuit and the downstream cooling circuit and ramping theircapacity based on the Call for Cooling, when the Call for Cooling isbetween SRD3 and SRD4, turning on the fixed and variable capacitycompressors of the tandem compressor of the upstream cooling circuit andramping the capacity of that variable capacity compressor based on theCall for Cooling percentage and turning on only the variable capacitycompressor of the tandem compressor of the downstream cooling circuitand running that the variable capacity compressor at full capacity, andwhen the Call for cooling percentage is greater than SRU4 and less thanor equal to SRU5 turning on the fixed and variable capacity compressorsof the tandem compressor of the upstream cooling circuit and of thetandem compressor of the downstream cooling circuit, running thevariable capacity compressor of the tandem compressor of the upstreamcooling circuit at full capacity and ramping the capacity of thevariable capacity compressor of the downstream cooling circuit based onthe Call for Cooling.
 4. The cooling system of claim 3 wherein SRU1 is25%, SRU2 is 45%, SRU3 is 65%, SRU4 is 90%, SRD1 is 0%, SRD2 is 15%,SRD3 is 40% and SRD4 is 65%.
 5. The cooling system of claim 1, whereinwhen there is an unmet Call for Dehumidification, the controllercontrols the tandem compressors based on the Call for Cooling and whichof a plurality of dehumidification control ranges that the Call forCooling falls including determining which of the variable capacitycompressors to ramp and controls the ramping of each variable capacitycompressor being ramped based on a the Call for Dehumidification andwherein control based on which of the plurality of dehumidificationcontrol ranges the Call for Cooling falls within and on the Call forDehumidification takes precedence when there is an unmet Call forDehumidification.
 6. The cooling system of claim 5, wherein thecontroller has Call for Dehumidification control thresholds that definethe plurality of dehumidification ranges that the Call forDehumidification can fall within including L1, L2 and L3 that progressfrom lower to higher values, the controller: when the Call for Coolingis between L1 and L2 turning on only the variable capacity compressorsof the tandem compressors of the upstream and downstream coolingcircuits and ramping their capacity based on the Call forDehumidification, when the Call for Cooling is between L2 and L3 turningon only the variable capacity compressor of the tandem compressor of theupstream cooling circuit and running it at full capacity and turning onthe fixed capacity compressor and variable capacity compressor of thetandem compressor of the downstream cooling circuit and ramping thecapacity of the variable capacity compressor of the tandem compressor ofthe downstream cooling circuit based on the Call for Dehumidification,and when the Call for Cooling is greater than L3 turning on the fixedcapacity compressors, ramping the capacity of the variable capacitycompressor of the tandem compressor of the upstream cooling circuitbased on the Call for Dehumidification and running the variable capacitycompressor of the tandem compressor of the downstream cooling circuit atfull capacity.
 7. The cooling system of claim 6 wherein L1 is 0%, L2 is45% and L3 is 65%.
 8. The cooling system of claim 7 wherein SRU1 is 25%,SRU2 is 45%, SRU3 is 65%, SRU4 is 90%, SRD1 is 0%, SRD2 is 15%, SRD3 is40% and SRD4 is 65%.
 9. The cooling system of claim 1 wherein theevaporators have cooling slabs arranged in an interleaved configurationwith one more coils of the cooling slab of the upstream evaporatorinterleaved with one or more coils of the cooling slabs of thedownstream evaporator.
 10. The cooling system of claim 9 wherein thecooling slabs of the evaporators each have a superheat section and2-phase section with the superheat section of the cooling slab of thedownstream evaporator arranged between the superheat section of theupstream evaporator and the 2-phase section of the downstreamevaporator.
 11. The cooling system of claim 1 wherein the downstreamcooling circuit includes a suction line heat exchanger.
 12. The coolingsystem of claim 11 wherein the upstream cooling circuit includes asuction line heat exchanger.